DRAFTBACKGROUND PAPER ON

ENERGY EFFICIENCY AND rrHE PULP AND PAPER INDUSTRY

Kenneth R. Gilbreath, Chesapeake Paper Products CompanyAshok Gupta, Natural Resources Defense Council

Eric D. Larson, Princeton UniversityLars J. Nilsson, Princeton University

Prepared for the ACEEE 1995 Summer Study on Energy Efficiency in IndustryGrand Island. New York, 1-4 August 1995

Contents

1. INTRODUCTION1.1. Role in the U.S. economy1.2. Energy use1.3. Environment

2. PROCESSES AND 'TECHNOLOGIES2.1. Pulping

2.1.1. Kraft (chemical) pulping2.1.2. High-yield pulping2.1.3. New pulping processes

2.2. Paper manufacturing2.3. Integrated pulp and paper mills

3. ENERGY USE AND EFFICIENCY3.1. Bleached krdft market pulp

3.1.1. Woodyard3.1.2. Digester3.1.3. Medium consistency processing3.1.4. Bleaching3.1.5. Pulp drying3.1.6. Black liquor evaporation3.1.7. Lime kiln

3.2. Newsprint fronl TMP3.3. Pressing and drying paper3.4. Technologies used throughout the industry3.5. Energy impacts of environmentally driven technology

4. BIOMASS BASED COMBINED HEAT AND POWER GENERATION4.1. Existing cogeneration technology4.2. Future cogeneration technology

4.2.1. Black liquor ga~ification

4.2.2. Bark and wood waste gasification4.2.3. Electricity export potential from kraft pulp mills

5. WDUSTRY ENERGY POLICY AND DECISION MAKING5.1. Energy decision making in the industry5.2. Industry-utility cooperation in DSM programs5.3. Cogeneration and other on-site utility systems5.4. Industry-government cooperation5.5. Standards and ceo-labels

6. FUTURE DIRECTIONS

DRAFTBACKGROUND PAPER ON

ENERGY EFFICIENCY AND THE PULP AND PAPER INDUSTRY

Kenneth R. Gilbreath, Chesapeake Paper Products CompanyAshok Gupta, Natural Resources Defense Council

Eric D. Larson, Princeton UniversityLars J. Nilsson, Princeton University

lit INTRODUCTIONPulp, paper, and paperboard mills account for 95% of energy use in the U.S. paper and allied products industry (SIC26Y and about 12% of total manufacturing energy use in the U.S.

Paper is one of the few basic materials, the per-capita demand for which has not saturated in the U.S. (Williams etat, 1987). The increase in apparent per capita consumption2 averaged 1.8% per year 1960-1980 and 1.6% per year1980-1993 (AFPA, 1994). The U.S. has the highest apparent consumption of paper in the world, 313 kg in 1990.For comparison, consumption in 1990 in the United Kingdom, Brazil, and India was 164, 28, and 2.8 kg per capita,respectively (fable 1.1). Consumption in the U.S. has been projected by the USDA Forest Service to reach 413 kgper capita in 2040, corresponding to a 0.6% annual increac;;e from 1990.

Total apparent consumption in the U.S. increased 3.0% per year 1960-1980 and 2.6% per year 1980-1993 (AFPA,1994). For comparison, the increase in world and Western European consumption averaged 3.2% and 3.1 % per year,respectively, 1971-1990. World and Western European demand has been forecast to grow by 2.5% and 2.4% peryear, respectively, over the next decade (Gundcrsby and Rennel, 1992).

1$1 Role in the UoS$ economyThe U.S. has the world's largest installed pulp, paper, and paperboard production capacity, some 86 minion air-drymetric Lonnes (ADMT)3 per year in 1994, or about 30% of world capacity.4 In total there are 201 pulp producingmills (AFPA, 1994), many of which arc annexed to paper or paperboard producing units. Pulp production capacityis some 63 million ADMT per year. (The next biggest producers are Canada and Scandinavia, with capacities of29 and 25 million ADMT per year, respectively.) The U.S. has a total of 547 paper and paperboard mills.

The value of exports and imports in 1993 was $9.6 billion and $10.6 billion, respectively. Major imports werenewsprint, printing and writing paper, and bleached kraft pulp. Major exports included paperboard, waste paper, andbleached kraft pulp. In addition to traditional competitors in the world market such as Canada and the Scandinavian

1 The paper and allied products industry group (SIC 26) consists of five 3-digillevel SIC industries: pulp mills (SIC261); paper mills (SIC 262); paperboard mills (SIC 263); paperboard boxes and containers (SIC 265); and convertedpaper and paperboard products (SIC 267). In this paper we focus on the fJIst three of these--pulp, paper, andpaperboard mills, sometimes called primary mills. We refer at times to these as the pulp and paper industry.

2 Apparent per-capita consumption is domestic production plus imports less exports, divided by population.

3 ADMT is a standard measure of the mass of pulp or paper. One ADMT is 10% moisture and 90% dry substance.

4 For comparison, the approximate capacities of other big producers (in ADMT per year) are Japan (35.6), Canada(20.5) and the former West Germany (14.9) (FAO, 1990).

2

countries, the U.S. industry is facing increasing competition from low cost producers such as Chile, Indonesia, Brazil,and South Africa.

The value of shipments from the U.S. paper and allied products industry (SIC 26) was $129 billion in 1991, rankingit eighth among all U.S. manufacturing industries, and the value added was S58 billion. The value of shipments frompulp, paper, and paperboard mills was $54 billion, and the value added was $25 billion (AFPA, 1994).

New capital expenditures in the last decade have averaged 10.4% of revenues (AFPA, 1994). This makes paper andallied products the most capital intensive of the manufacturing industries. In addition, pulp, paper, and paperboardmills are more capital intensive than the paper and allied products industry average. Pulp, paper, and paperboardmills employed nearly 200,000 people in 1991 and had a payroll of about $8 billion.

Wood for pulping represents the largest cost among material inputs to the pulp and paper industry, accounting foran average of 21 % of total material and energy costs. The corresponding numbers for energy, wood pulp, andchemicals are 17%,15% and 6%, respectively, Table 1.2. It should be noted, however, that the industry uses abouttwice as much energy as indicated by these numbers since 56% of total fuel and electricity use is self generated(primarily from spent pulping liquors, wood residues, and bark) and thus does not appear explicitly as a purchasedenergy cost.

The capital intensity of the industry and associated scale economies have contributed to the closing of many smallerpulp and paper mills. The number of mills in the U.S. fell from 635 in 1980 to 544 in 1990 and most of theshutdowns were of mills that produced 50,000 ADMT per year or less. Mills with a capacity of 500,000 ADMTper year or more grew from 20 to 40 in the same time period (Dils, 1992). However, partly because of thedifficulties of siting very large mills and partly because of the geographic distribution of waste paper there is nowa trend toward the building of smaller mini-recycled pulp mills near urban areas. The equipment for processingrecycled fiber is more modular and thus does not offer the same economies of scale that characterize virgin fiberpulp mills.

Over the past several years, U.S. pulp and paper companies spent about 1% of sales on research and development.For comparison, the automotive and chemicals industries spend more than 4% of sales and the overall average forthe industry in 1991 was about 3.6% (Dils, 1992). Companies that supply the pulp and paper industry withequipment, chemicals, elC., have taken a more central role in conducting and sponsoring research than the industryitself.

I1D249 Energy useThe energy intensity (expressed in megajoules per $ value of shipments) of the paper and allied products industrywas 21 MJ/$ in 1991, ranking it as the second most energy intensive industry group in the manufactwing sector(Figure 1.1). It accounts for 12.4% of total manufacturing energy use, or about 3% of national energy use. At the4-digit SIC level, the energy intensity of paperboard, pulp, and paper mills puts them in 4th, 6th, and 12th place,respectively (Figure 1.2). Total energy use in the pulp and paper industry in 1991 has been estimated to be 2.5 EJ(MECS, 1994), corresponding to about 95% of total energy use in the paper and allied products industry. Forcomparison, the American Forestry and Paper Association reports similar numbers based on member surveys: 2.7EJ in 1991 and 2.8 EJ in 1993 (Table 1.3).

The industry has made important strides in reducing loud energy use since 1973 and in increasing the fraction ofenergy provided from self-generated biomass sources. Total energy use per ADMT decreased by about 1.5% peryear between 1972 and 1986 and has remained relatively constant in recent years (Fig. 1.3). Use of fossil fuels andother purchased energy decreased by about 30/0 per year between 1972 and 1986 (from about 25 GJ per ADMT in1972 to 17 GJ per ADMT in 1986) and subsequently leveled off.

Figure 1.4 shows estimates of the major energy flows in the industry, based on data from the Manufacturing EnergyConsumption Survey (MECS, 1994). The major end uses of process steam are digesters, evaporators, and driers.The major electricity use is for machine drives, which arc primarily for pumps, fans, and wood grinding equipment.

3

The ratio of electricity-to-heat demand has been increasing steadily in the pulp, paper, and paperboard industry. Thistrend is reflected, for example, in a near doubling of purchased electricity between 1972 and 1993, while totalpurchased energy decreased by 7%. The trend towards higher electricity to heat ratios is being driven by increasingelectrification (in part to meet more stringent environmental regulations), combined with thermal energy efficiencyimprovements. As a result there is growing interest in the industry in electricity conservation and in cogenerationtechnologies characterized by higher electricity-to-heat production ratios.

The pulp and paper industry can be distinguished from most other manufacturing industries by its extensive use ofbiomass fuels and cogeneration. Self-generation of energy amounted to 56% of total industry energy use in 1993(Table 1.3), up from 40% in 1972 (Fig. 1.3). The most recently available survey data indicates that about 50 TWhof electric power was cogenerated in the industry in 1991 (API, 1992), which is equivalent to about half of totalelectricity consumed.

1.30 EnvironmentEnvironmental concern, manifested in changing market demand and more stringent environmental regulation, isamong the most important drivers of technological change in the pulp and paper industry. The industry's annualcapital expenditures on environmental protection between 1987 and 1993 was estimated to be between 10% and 19%of total annual capital expenditures (AFPA, 1994). While requiring capital expenditures, environmental concernshave also provided a competitive advantage for the industry against paper substitutes, since the industry producesrecyclable products from renewable resources using relatively few non-renewable inputs.

Environmental issues associated with pulp and paper manufacture include solid waste disposal; emissions to waterof chlorinated organic compounds or adsorbablc organic halogens (AOX), including dioxins and furans, chemical/biological oxygen demand (COD/BOD), phosphorous, and niLrogen; and emissions to air of sulfur and nitrogenoxides from fuel combustion.

The industry has made some important strides in reducing effluents from mills. S02 emissions in 1990 from pulpand paper mills were estimated to be 544 thousand tannes, 75% of which were from fossil fuel fired boilers. Thisreptesents 2.6% of the national total S02 emissions of 21.1 million tonnes. Emissions of NOx (277,000 tonnes)represented 1.4% of the national total in 1990. Between 1980 and 1990, a period during which paper productionincreased by about 30%, the emissions of S02 decreased by 30%, primarily as a result of switching to lower sulfurcoal and oil, reduced oil consumption, and the installation of desulfurization equipment. NO" emissions increasedby 10% in the same period (NCASI, 1993). The amounts of BOD disc~arged in final effluent per unit of productionin 1988 was one-third to one-quaner of that discharged in 1975 (NCASI, 1991).

The emissions from bleaching operations are considered the most toxic (Paulsson and FaUenius, 1987), and theenvironmental impacts of AOX have generated considerable debate. AOX is a gross measure of aU chlorinatedorganic compounds and is often used as a measure because lesting for it is inexpensive and reliable. AOX emissionsare primarily associated with the kraft chemical pulping process, where chlorine or chlorine dioxide is used as ableaching agent.

Environmental regulation to reduce AOX emissions and market demand for chlorine free products have driven thepulp and paper industry to find alternatives to elemental chlorine (Clz} as a bleaching agent. Chlorine dioxide (CIOz)bleaching gives much less AOX than elemental chlorine bleaching and decreases the relative amount of the highlychlorinated and potentially most toxic compounds in the AOX (SEPA, 1992). Swedish data show that typical AOXemissions from bleach plants using elemental chlorine in the 1970s were 8-10 kg/ADMT (SEPA, 1992). Emissionsfrom U.S. pulp mills have not been measured as consistently but are likely to have been in the same range.Emissions today are typically around or below 1 kg/ADMT froln pulp mills in Scandinavia. Many modem NorthAmerican mills reach the same levels.

There has been no federal or state regulation for AOX emissions although some individual mills have been pennittedfor AOX at levels similar to those faced by mills in other countties, e.g., 1-2 kg/ADMT for kraft pulp mills atpresent, and less in the longer term (EPA, 1993). The "cluster rules" which have been proposed by the EPA andthat are expected to be promulgated in 1996 cover both air and water emissions and include AOX (Hinsey, 1994).

4

Elemental chlorine free (ECF) and totally chlorine free (TCF) pulp accounted for an estimated 70% and 30%,respectively, of the bleached chemical pulp production in Finland and Sweden in 1994, up from 25% and 0%,respectively, in 1990. The production of TCF in North America is negligible. ECF pulp was estimated to have a25% and 50% market share of bleached chemical pulp production in the U.S. and Canada, respectively, in 1994(NLK, 1994). The switch to TCF in Scandinavia has been driven largely by consumer demand for TCF paper inwestern Europe. At the current AOX levels from chlorine dioxide bleaching « 1 kg/ADMT), it has not beenpossible to predict the environnlcntal impact of an effluent on the basis of its AOX content, in part because non­chlorinated compounds in the mill effluents can contribute a non-negligible effect (AxegArd et aI., 1993). Since TCFbleaching is relatively new, there are not sufficient data to draw finn conclusions about the environmental impactand toxicity of discharges from TCF pulp mills. Nevertheless, consumer demands arc likely to continue to push theindustry toward greater use of TCF bleaching.

Environmental concern has also led to increased paper recycling. The solid waste management dilemma in keyregions of theV.S. has been especially important in motivating a substantial increase in recycling. It has beenestimated that 37.5% (by weight) of all municipal solid wasLe in 1990 was paper and paperboard (EPA, 1992). Itis expected that investments in equipment to utilize recycled paper will account for the largest share of capitalexpenditures in the industry in the next few years (MelZ, 1994). The fraction of new paper supply that is recoveredfor recycling has increased from 22% in 1970 to 39% in 1993. More than half of the increase occurred between1988 and 1993. The industry intends to increase the fraction to 50% by 2000 (AFPA, 1994c). The present fractionin leading recycling countries like Japan, Austria, and the Netherlands is slightly higher than 50% (Bryntse, 1992).

5

Table 1.1. Apparent per capita paper and paperboard consumption in selected countries. Source: FAO, 1993; Bureauof the Census, 1994.

Country 1980 cons. 1990 cons. increase 1980-90 annual average total cons. 1990(kg/capita) (kg/capita) (kg/capita) increase (%) (million tons)

USA 267 313 47 1.6 78

Sweden 209 250 41 1.8 2.1

United Kingdom 121 164 43 3.1 9.4

France 115 154 39 3.0 8.7

Brazil 28 28 0.4 0.1 4.2

India 1.8 2.8 1.0 4.5 2.4

6

Table 1.2. Production cost breakdown in million U.S. dollars in the pulp, paper, and paperboard industry in 1987(Source AFPA, 1994a).

Category Pulp mills Paper mills Paperboard mills AU primary(SIC 2611) (SIC 2(21) (SIC 2(31) mills

Total mtrls & energy 2,019 14,856 6,840 23,715

Pulpwood 752 2,212 2,016 4,980

Chemicals 221 802 352 1,376

Wood pulp o.a. 3,538 51 3,589

Waste paper d.w. 541 969 1,511

Other materials 782 5332 2110 8,223

Energy 265 2,431 1,342 4,037

Value added 2,281 14,024 6,914 23,220

Employee costs 660 5,641 2,230 8,531

Contrib. to overhead 1,622 8,384 4,684 14,689

Value of shipmentsa 4,313 28,918 13,730 46,961

n.3. = not applicabled.w. =data withheld by Census of Manufacturers to avoid disclosing data for individual companies(a) Value of shipments is not exactly equal to materials & energy plus value added due to inventory changes.

7

Table 13. U.S. pulp, paper, and paperboard industry estimated energy use in 1993 (Source AFPA, 1994b)

Source Estimated Use Percent of total(thousand Gigajoules)

Purchased electricity· 181,878 6.6

Purchased steam 34,424 1.2

Coal 363,104 13.1

Residual fuel oil 186,715 6.7

Distillate fuel oil 6,980 0.2

Liquid propane gas 3,152 0.1

Natural gas 466,280 16.8

Other purchased energy 13,658 0.5

Energy sold -38,230

Total purchased fossil fuel & energy 1,217,959 44.0

Hogged fuel 235,352 8.5

Bark 170,686 6.2

Spent liquor 1,108,689 40.0

hydro electric powe~ 15,282 0.6

Other self-generated 21,477 0.8

Total self...generated and residue fuels 1,551,486 56.0

Total energy 2,769,445 100

(a) Electricity has been converted from kWh Lo GJ using a 3.6 MJ/kWh conversion factor.

8

SIC 29, Petroleum and Coal Products (29.50/0)

SIC 26, Paper and Allied Products (12.4%)

SIC 33, Primary Metal Industries (12.2%)

SIC 28, Chemicals and Allied Products (24.9%)

SIC 32, Stone, Clay and Glass Products (4.3%)

45

40

~ 350en:;)

" 30...,::~

>a 25~encG)..,

20c:->een 15"'"CDCw

10

5

0

0 500 1000 1500 2000 2500 3000

Value of Shipments (billion US$)

1.1. Energy intensity for different industry groups at the 2-digit SIC level plotted against the value of shipmentsin 1991. Numbers in parentheses are percentages of total manufacturing energy consumption (MECS, 1994; ASM,1992).

9

SIC 2631 Paperboard Mills (4 240/0),-~

./ SIC 2611, Pulp Mills (1 ~48%)

~~

~

-SIC 2621, Paper Mills (5~

- Q) ~Q)

It~.2

(jjc::: -cccs c:::en ......-.. ccs- Ii-~ en0 0 E('I) Q)

~co ::J U 0 -.,... Q)~ n3 <.0";:: .,..... e c C\J..- "'-'"0 ....

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~ 50~

en:>"'"..,~ 40~

.>a~(/)

30c(1).,C

~ 20C")Irmm.

CUCW

1 0

60

o 100 200 300 400 500

Value of Shipments (billion US$)

Figo 1~2$ Energy intensity for different industries at the 4-digit SIC level plotted against value of shipments in 1991.Numbers in parentheses are percentages of total manufacturing energy consumption. Paperboard mills are surpassedin energy intensity only by lime (179 MJ/$), nitrogenous fertilizers (174 MJ/$), and cement (89 MJ/$). Industrieswith energy intensities above 20 MJ/$ account for 74% of total manufacturing energy use but only 13.3% of the totalvalue of shipments (IvlECS, 1994; ASM, 1992)"

10

50

~:s 30c~ci 20

(primary equivalent)10 ....1--------------------------.--------.-----.----

o1970 1975 1980 1985

Year

1990 1995

Fig'& 1,,340 Total primary energy and purchased energy use in the pulp, paper, and paperboard industry. Purchasedelectricity and steam have been converted to their primary energy equivalents using 10.8 MJ/kWh and an 85% boilerefficiency, respectively. Source: Lisa Wolfe, American Forestry and Paper Association, personal communication,Washington, DC, May 1995.

11

Purchased 187 - 347Electricity -1\

9161

Unrecovered

Electricity 32\)..,a.ste Heat

Sold.:

I~

193 2021

"' 1954Electricity from

9 Onsite Electricity 141........-

~Noncombustible Generation

Vlaste Heat-S!2..Renewables

325606

I

13 312

J "-Boiler 1934Process

611=0.71 ~

Healing-Eo--

~ 2095

1187

r ""\ 'II0 Process 3

Net 41~ Cooling and ~-Steam 1228 Refrigeration

2108

I~ I~ ;~ J J 1"\ 681\1achine

296136 ~

Drive~

Residual 159 '\'\\\\\ 23 -Fuel Oil

... 7 -0 4~

Electro-Chemical~

2 Processes

Distillate 5 :; 3 ::::-Fuel Oil ...J 24 4

~Other~

385 Process Use

\ II503 118 _ - 2056"

-- -------- -3381\atural Gas :;;..J - 149 1377

25 11~

Facility~

I HVAC

LiquifiedPetroleum

3 2 -- ...J -Gas 0 9

~Facility~

306 Lighting

Coal308 2 ::- J 0 3

~Facility~

0 Support

Coke and 0 - 0 -Breeze ~

-3 1~

Onsite~

1278 Transportation

Other Energy1278 0

Sources Except -Net Steam 2 Other 0

~ Non-Process ~Use

Fig.. 1.4$ Estimated energy flows (PI per year) in the pulp, paper, and paperboard industry in 1991. Source: BarryHyman, Dept. of Mechanical Engineering, University of Washington, personal communication, Seattle, WA, May1995. See also Giraldo and Hyman (1994).

12

2. PROCESSES AND TECHNOLOGIESAs background to subsequent discussions on energy use, this chapter gives a brief introduction to the basic principlesof pulp and paper making.

201. PulpingPulping is the process by which the fibers in the wood are separated and treated to produce a pulp. The wet pulpis converted into paper at an integrated pulp and paper mill, or is dried and transported from the pulp mill to apapermill. Different pulping processes are used depending on the raw material and the desired end-product. Theprocesses can be categorized as chemical, mechanical (also called high-yield), semi-chemical, and chemi-mechanical.There are additional processes for extracting fiber from recycled paper.

The predominant process is the chemical process, kraft (also called sulfate) pulping. Kraft pulp accounts for about80% of all pulp produced in the U.S. (Fig. 2.1). In kraft pulping the ligninl is dissolved in a digester wherewoodchips are cooked. The pulp yield, i.e., the mass of pulp dry substance produced divided by the mass of wooddry substance input, for bleached and unbleached kraft is about 40-50% and 50-65%, respectively. The energycontents of the dissolved lignin and other organic compounds is used to produce heat and electricity for the process.

In high-yield pulping the wood is subjected to shear and compression forces in order to separate the fibers. Themechanical processes have yields above 90% but they produce a pulp which makes a relatively weak paper partlydue to the damage done to the fibers in the process. The paper ages relatively rapidly due to the presence of lignin.

Semi-chemical and chemi-mechanical pulping, as the names imply, involve a mix of chemical and mechanicaltreatment and typical yields are 65-85%, and 85-95%, respectively. Recycled paper is primarily mechanically pulpedand may also require deinking. The fiber yield from recycled paper is typically 70-90%.

Most pulp is pumped as a slurry to a paper or paperboard plant where it may be mixed with other pulps, recyclefiber, or fillers such as clay, before going to the paper machine. Only about 10-15% of U.S. pulp production is driedand sold as market pulp. Most (95%) of market pulp is bleached or semi-bleached kraft (AFPA, 1994).

Mechanical pulps are used, for example, to make newsprint and other printing and writing papers, sometimes withthe addition of chemical pulps to improve strength and other qualities, Figure 2.2. Another application is flutingmaterial, Le., the corrugated middle layer in conlainerboard sandwiched between layers of linerboard. Bleached kraftis used primarily in high quality printing and writing papers. Unbleached kraft pulp is used for producing linerboardand sack paper.2 Semi-chemical and chemi-mechanical pulps are typically used in boxboard and sanitary products.Recycle fiber can be used in a broad range of products, e.g., boxboard, fluting, newsprint and sanitary tissue. Thebiggest paper and paperboard products are printing and writing papers, and containerboard (Fig. 2.3).

2$1 ..1.. Kraft (chemical) pulpingAs noted earlier, kraft pulping is the dominant pulping process in the U.S. (Fig. 2.1). The name kraft has its originin the Gennan word for strength, reflecting an important characteristic of this type of pulp. In the kraft process, amixture of sodium hydroxide (NaOH) and sodium sulfide (Na2S) in an alkaline solution with a pH of 13-14 is usedto pulp the wood. An alternative chemical process to kraft pulping is sulfite pulping, which uses hydrogen sulfite(HS03), or sulfite (503

2-), as the active chemical in an aqueous acidic or neutral solution. In either the kraft or

sulfite process, woodchips are impregnated wi~ the pulping liquor and then heated under pressure for a few hoursin a digester to dissolve the lignin. The fib,--J's are separated from the spent pulping liquor from which processchemicals and energy are recovered in both the kraft and the sulfite processes. Additional discussion here is limited

1 The main chemical components of wood are cellulose, hemicellulose, and lignin. The lignin acts as a binder forthe wood fibers.

2 Different wood species have different properties which make them suitable for different products. For example, themean fiber length of softwood species is usually 1-5 Inm whereas it is 0.5-2 mm for hardwood species (Nordman,1989). The longer fibers make softwood a good raw material for strong papers such as sack and linerboard.

13

to kraft pulping.

The fiber line in the kraft process (Grace and Malcolm, 1989) is shown schematically in Figure 2.4. Following thedebarking and chipping of the pulpwood, the chips are screened for size before being fed to the digester. Thedigestion may be batch or continuous. The wood chips are first treated with steam (in the presteaming vessel) todrive air from cavities and replace it with steam. When the chips meet the somewhat colder mixture of pulpingchemicals--white liquor (NaOH + Na2S)--at about 80..900C, the steam condenses, creating an underpressure thatfacilitates impregnation of' the chips with white liquor. The chips are heated for 1-1.5 hours up to the desiredcooking temperature, where they remain for another one hour or more. The target temperature is usually 165-175°C.Higher temperatures increase chemical reaction rates, but above about 180OC, pulp quality degrades and steamdemand increases.

After the digester, the fibers are separated from the spent pulping liquor through several stages of countercurrentwashing. Heat is recovered from the digesters as condensate and flash Sleam. The pulp is then screened, oftenbleached, and then pumped to the paper mill or dried before shipping it from the mill.

Bleaching increases the brightness of the pulp by decolorizing, degrading and dissolving the colored components.It is done in several steps using oxidants to degrade and dccolorize lignin and sodium hydroxide to degrade lignin(by hydrolysis) and aid in its dissolution (Reeve, 1989). The principal oxidants and (in parentheses) their commonidentifier in the industry, are chlorine (C), chlorine dioxide (D), oxygen (0), hypochlorite (H), hydrogen peroxide(P), and ozone (Z). Sodium hydroxide is identified by the symbol E, for caustic extraction. A common bleachingsequence in the 1980s was CDEoDED, where the subscript refers to the chemical used to augment the primarychemical in a particular stage. Modem pulp mills use oxygen delignification Lo remove more lignin after digestionand before bleaching, thereby reducing the amount of bleaching chemicals required and the bleach plant effluentemissions. Chlorine dioxide (CIOJ has been substituted for chlorine (CIJ in modem bleach plants due to the largeemissions of chlorinated organic compounds in the waste water associated with the use of elemental chlorine (seeSection 1.3). The demand for totally chlorine free (TCF) paper, primarily in the European market, has led to theincreased use of bleaching agents such as hydrogen peroxide and ozone, and development of alternative bleachingtechnologies.. A typical TCF sequence today might be oxygen delignification followed by ZEoP.

The spent pulping liquor--weak black liquor....is evaporated to increase the solids content (organic compounds andpulping chemicals) from 10-20% to 60-750/0 and then goes to the "recovery" section of the mill. It is burned therein a Tomlinson recovery boiler (Fig. 2.5). Steam generated in the boiler is used to produce electricity and processsteam in a turbine with steam extraction at low and intermediate pressures. (See Section 4 for additional discussion.)The pulping chemicals are recovered from the bottom of the boiler as a smelt of sodium sulfide (N~S) and sodiumcarbonate (NazCO)) which is then dissolved in water to form green liquor. The green liquor reacts with an aqueoussolution of calcium hydroxide (CaOH~ (formed by mixing calcium Qxide--CaO, or lime--with water) in thecausticizer, converting the sodium carbonate to sodium hydroxide and thereby regenerating white liquor for thepulping process. The precipitate from the causticizer, calcium carbonate (CaC03)--called lime mud--is burned in alime kiln to regenerate CaO.

201~2~ HighmoyieBd pulpingThe main high yield, or mechanical, processes are stone groundwood pulping (SGW) and thermo-mechanical pulping(TrvIP). Refiner mechanical pulping (RMP) is another important process that was introduced in the 1960s, but whichis now increasingly being substituted by TMP. In general, thermomechanical pulp is stronger than refiner mechanicalpulp which in tum is stronger than stone groundwood pulp (Malinen, 1989). Stone groundwood pulp was the onlymechanical pulping method used for paper production before the introduction of RMP technology in the 1960s. Inthe SGW process, debarked logs are pressed against a large rotating grindstone which separates the fibers (Fig. 2.6).The fibers are continuously washed off the stone, and the washing maintains the stone tempemture below aboutl000C.. The resulting slurry is screened, and oversized pieces are fed to a reject refiner or grinder. Stonegroundwood pulping, which has traditionally been done at atmospheric pressure, is being replaced with pressurizedstone groundwood (pGW) pulping where the grinding zone is kept at an elevated pressure. This facilitates therecovery of higher grade heat (in the form of low pressure steam).

14

Refiners were originally used to produce wood for fiberboard or chipboard from low-quality wood such as crookedlengths of wood, wood thinnings, and some types of chips from sawmills. The technology was developed to producepulp for paper manufacturing during the 1950s. Refiner pulping is less sensitive to the quality of the feed than stonegroundwood (Borg, 1989). Refiners are designed with a single rotating disk against a fixed disk (single-disk refiner)or with two rotating disks (double-disk refiner) (Fig. 2.7). Individual refiner units have from 0.2 MW to 20 MWof installed electric motor power (Gavelin, 1991).

In thermomechanical pulping, the chips are preheated and softened by steam and the refiner is pressurized to facilitatesteam recovery. The pulp is screened to remove knots and shives (large fiber bundles) before going to the papermill.

The bleaching technology used for mechanical pulps is different from chemical pulps since the lignin is preserved.Pulp for making newsprint usually does not need any bleaching if fresh and moist chips from spruce are used.Ditionite (Na"S204) or hydrogen peroxide (H20z) are typically used as bleaching agents if some increase in brightnessis desired.

Wastepaper is also pulped, or repulped, mechanically. Wastepaper is mixed with water, agitated to tum it into aslush, and screened to remove metal, plastic, etc. Chemicals and enzymes can be added to remove ink and the pulpmay be bleached.

2.1.3. New pulping processesChemical pulping processes under development include sulfur free processes, some of which use organic solvents,and alkaline sulfite processes. Environmental concern associated with kraft pulping is an important motivation fordeveloping these processes since they are free from odorous reduced sulfur compounds and may facilitate bleaching(Tulenheimo et al., 1992; Stockburger, 1993).3 Another advantage of some sulfur free processes is that chemicalsrecovery can be simpler than for kraft pulping. As a result they can be less capital intensive and sensitive toeconomies of scale, making smaller size mini ...mills feasible (Dils, 1992). Examples include the AlceH process inCanada, which uses ethanol as the pulping chemical, and the Organocell process in Germany, which is based on analkaline cooking liquor with 25-30% methanol (Lora ct aI., 1993; Stockburger, 1993). The Alcell process has beenoperated on a pre-commercial scale at 15 ADMT/day and the Organocell process on a commercial scale at 400ADMT/day. Other sulfur free processes include a Russian alkali-oxygen process, and the Finnish Milox processwhich uses peroxyformic acid as a pulping agent (Tulenhcimo et at, 1992).

The ASAM (alkaline sulfite anthraquinone methanol) process is another noteworthy process which is based on thecommercially proven sulfite process but has improved environmental characteristics and is more flexible with regardto the wood feedstock (Hentges et aI., 1993). A 5 ADMT/day pilot plant has been operated at a mill in southernGermany. Potential disadvantages of ASAM when compared with kraft pulping include a more complicated chemicalrecovery system, higher chemical costs, and higher capital costs (Stockburger, 1993).

One of the reasons that the ASAM and other new processes have been difficult to commercialize is the expectationthat the kraft process will be able to meet future stringent demands on environmental performance. Also, becauseof the capital intensity and yct-to-bc-commerciaJ Iy proven nature of such processes, risks in adopting them arerelatively high.

In mechanical pulping there has been some interest in developing explosion pulping, a process which has been knownsince the 1920s but which has been used primarily only for fiberboard manufacture. The principle of explosionpulping is to heat impregnated woodchips at very high pressure and then blow the chips into a vessel of lowerpressure where the sudden expansion of steam and gases tears up the chips and produces a coarse fiber. Explosionpulping, as an alternative to chemi-thermomechanical pulping for producing fluff pulp, has been estimated to requireone-third the power (Tulenheimo et aI., 1992).

3 Based on our review of the literature, it appears thal these new chclnical pulping processes will have energydemands that are similar to the kraft process since they include the major energy using unit processes, i.e., digesters,chemicals recovery, and pulp (or paper) drying.

15

2.2. Paper manufacturingThe production of paper involves preparing the stock from pulp, fonning a sheet, dewatering and drying, andsometimes coating the paper. The stock is often prepared from a mix of different pulps sometimes with the additionof mineral fillers, depending on the desired paper quality, Figure 2.2. For example, newsprint can be made from amix of mechanical pulp and recycled pulp, with some addition of chemical pulp to increase strength. Chemical pulpsare usually refined or beaten to improve fiber bonding and facilitate the forming of a strong paper.

All paper machines have three basic elements, wet end (gravity removal of water), press section (mechanical pressingto squeeze out water), and drying section (evaporation of water) (Fig. 2.8). The stock has a solids content (orconsistency) of less than one percent when it is sprayed onto the wire mesh at the wet end for dewatering. Mostof the water is removed in the wet end by gravity and vacuum suction boxes. The consistency approaches 20% whenthe sheet enters the press section where the water is squeezed out by pressing the web between cylinders until theconsistency is 40-50%. In the drying section the remaining water in the sheet is removed by evaporation and thepaper is dried to 90-95% through contact with steam heated cylinders. The paper may also be coated after dryingto make it smoother or brighter.

The papermachine used depends on the type and quality of the paper produced. For example, double-wire machines,where the stock is sprayed between two wire meshes, are often used for newsprint and tissue, whereas sack and linerare made using single-wire machines. The drying section may be a multicylinder machine with 40-100 dryingcylinders of 1.5-1.8 m diameter (for example used for newsprint and sack), or a 4..7 m diameter yankee cylinder (forexample, used for tissue), or a combination of both (Fig. 2.8).

Economies of scale have resulted. in larger and faster paper machines. For example, newsprint machines run at1,000-1,500 meters per minute and are about 8 m wide. Future machines will be designed for speeds and widthsapproaching 2,000 meters per minute and 10 meters, respectively (Sobczynski, 1994). The corresponding productionrate for a single large newsprint machine would be over 1000 ADMT per day. However, there is a parallel trendtoward low cost, simple, and small paper machines for mini-recycled paper mills.

2$3 Integrated pulp and paper millsPulp and paper mills arc often large and complex facilities that may produce several pulp and paper qualities fromboth softwood and hardwood feedstocks. The mix of products and qualities complicates comparisons betweendifferent pulp and paper mills and their energy use. As an illustration, the Chesapeake Paper Products Companyoperates a 2,000 ADMT per day containerboard, kraft paper, and bleached hardwood pulp mill in Virginia. Majorproduction facilities at the min include 8 batch digesters for virgin kraft pine fiber, a continuous digester for virginkraft hardwood fiber, a 570 ADMT per day hardwood pulp bleach plant, a bleached hardwood market pulp machine,a 900 ADMT per day recycled fiber plant, a corrugating medium machine, a linerboard machine, and a three-plyIinerboard machine. Other major facilities include 2 chemical recovery boilers, a wood waste boiler, a coal-firedboiler, two oil-fired boilers, and 7 steam generators. The implication of this diversity is that many of theopportunities for end-use energy efficiency improvements and cogeneration are mill specific.

16

United States Pulp Production 1993(Total: 57 Million ADMT)

Bleached and Semi­Bleached Kraft

~ Unbleached Kraft

mSemi-Chemical

~ Thermomechanical

• Groundwood and Refiner

Sulfite

Fig. 2.1. U.S. pulp production by process in 1993 (AFPA, 1994a).

17

Paper and Paperboard CompositionLINERBOARD - Made of one or more layers of chemical pulp

UNCOATED FREESHEET (Copy Paper) - Made of chemical pulp and mineral fillers

mineral fillers, and

Mineral Filler

~

l\Aechanical pUlp Chemcial pulp Coating

18

2~2. Typical paper and paperboard compositions. Source: R.E. Buttner, H.A. Simons, Vancouver, Be, Canada.

United States Paper and Paperboard Production 1993(Total: 77 Million ADMT)

Printing & Writing

~ Containerboard

mBoxboard

mOther Paperboard

• Newsprint

1.1 Tissue

II Packaging and Industrial

2030 U.S. paper and paperboard production in 1993 (AFPA, 1994a).

19

CONTINUOUSDIGESTER

WOODCHIPS

WHITELIQUOR

SCREENING1WASHING

WATER

OXYGENDELIGNIFICATION

Cl

ALKALI ALKALI

y

PULP DRIER

20

Fjg~ 20~t Kraft pulping fiber line (Borg, 1989).

PROCESS STEA.~

STRONGBLACKLIQUOR

DISSOLVER

WEAKLIQUOR

RECOVERYBOILER

Fig.. 2$5.. Kraft chemicals recovery cycle (Borg, 1989).

21

SHAU'UHNG

&.A'WE,

2.Ji~ Grinder for stone groundwoood (SGW) pulp production (Eliah and Lowin, 1988). A recently sharpenedpulpstone gives a coarser pulp, so a SGW pulp mill typically has many units from which the pulp is mixed tomaintain uniform quality.

22

CHIPS

• i

=1=1----._-= II==:::-- ------- ---=== ====,-- --

Fig0 2..7" Double disc refiner (Borg, 1989). The gap between the discs at the periphery is only about 0.1 mm. Arefiner typically operates at 1500-1800 rpm ..

23

DRYING SECTION YANKEECYLINDER

Fig.. 2~8o Example of a papennachine with a single-wire wet end, and with both multicylinder and yankee cylinderdrying (Borg, 1989).

24

3. ENERGY USE AND EFFICIENCYThe Manufacturing Energy Consumption SUlVey (MECS, 1994) gives estimates of energy use in the pulp, paper andpaperboard sectors of the paper and allied products industry. However, the wide variety of processes and productswithin each of these sectors makes it necessary to analyze energy use at a more disaggregated level to understandthe underlying trends that affect energy use, as well as the potential for energy efficiency improvements. Energyuse in basic materials industries is typically expressed in energy per unit of product. However, such comparisons arecomplicated by the fact that pulp and paper mills that produce more than one product often do not allocate energyuse by product. In addition, different mills may not use the same energy accounting principles or be able to producethe same breakdown of energy use by unit process. Moreover, most mills also consider energy use data proprietary.

It is possible to get some indication of relative energy performance across the industry by focussing on single-productfacilities. In this section we focus on two of the most important pulping processes: kraft and thermomechanical.We discuss drying energy use in the context of newsprint, but the discussion is generally applicable to a variety ofproducts. We also discuss some energy end-use technologies that are used throughout the industry, e.g., motors andpumps, and the potential energy impacts of environmentally-driven technology changes. Some of the data presentedare from our own energy audits and some arc drawn from other sources.

3.1. Bleached kraft market pulpBleached and unbleached kraft pulping processes arc essentially the same except bleached kraft is cooked to achievea higher level of delignification in the digester, and the pulp is bleached. The pulp is then dried in a market pulpmill, whereas it is converted into dried paper at an integrated mill. As in most industries, new or modernized plantstypically use less energy than old plants. The industry has been more effective in reducing steam demand thanelectricity demand in new and retrofitted mills, which has contributed to the trend of increasing electricity to heatratios (Table 3.1).

The major steam users in a kraft pulp mill include the digesters, the evaporators, and the pulp drier (or, alternatively,the paper machine in an integrated mill) (Fig. 2.4 and 2.5). These three unit processes account for about three­quarters of total steam use in most market pulp mills.

Electricity demand is more evenly distributed throughout the min than steam demand. Most electricity is used inpumps and fans: 40-450/0 and 15-200/0 of the total, respectively, according to a detailed electricity audit of twoSwedish mills (Axelsson, 1990). Most of the installed fan capacity is in the dryer, lime kiln, and boilers. Majorelectricity using machine drives are found in the woodyard and the pulp screening plant. Other electricity uses arepresent at an integrated min: stock preparation and the paper machine may each account for about 200-300kWh/ADMT of electricity demand in a linerboard mill (Warnquist, 1989).

Table 3.2 presents a breakdown of energy use for two U.S. mills and two hypothetical model mills in order toillustrate the development in energy use over time and to give an indication of the potential for efficiencyimprovements. The U.S. mills are described in more detail by Larson (1992) and Subbiah et a1. (1995). The twomodel mills represent what a major Swedish consulting firm considers possible using 1980-vintage and 2000-vintagetechnology in a greenfield mill. l Energy use in unit processes is discussed in more detail below, with reference toTable 3.2.

3..101& WoodyardThe main woodyard operations typically include debarking, chipping and conveying, for which electricity demandis about 10, 15, and 20 kWh/ADMT, respectively (Jonsson et aI., 1977a). Some pulp mills buy chips from nearbysawmills. There is virtually no heat demand except in cold climates where logs must be thawed before debarking,something which may be done using low level waste heat. Modem plants use dry debarking to avoid generatingwastewater with high BOD/COD and increasing the moisture content of the bark which is used as fuel. Chippersequipped with adjustable speed drives to control cutting speed can save electricity might typically be adopted because

1 The primary objcctive for the model mill work has been to develop input to the Swedish government long-termenergy planning.

25

they improve chip and pulp quality (Aberg and Eriksson, 1987). For chip transport under full load conditions, beltconveyors would use only 0.5 Wh per tonne-meter (Wh/t-m) (horizontal) and 5 Wh/t-m (vertical)2 compared to 15Wh/t-m and 10 Wh/t-m for pneumatic and screw conveyors, respectively (Nygaard, 1986).3 Belt conveyors do lessdamage to the wood chips, in addition to using less energy, and are generally preferred over pneumatic conveyors.

3.1.2. DigesterKraft pulping can be done in a batch or continuous process. A particular driving force in kraft pulping technologydevelopment has been the need to reduce emissions of chlorinated organic compounds from the bleach plant. Forthis purpose pulping processes have been modified to reduce the need for bleaching by cooking to lower kappanumber (Le., lower lignin content)4 and by using oxygen delignification before the bleach plant.

Modified cooking or extended delignification can be achieved in both batch and continuous digesters. The basicprinciple is more selective delignification by controlling primarily the alkali concentration during the cook. Forexample, in conventional continuous digesters all the alkali--white liquor--is charged at the beginning of the cook.In modified continuous cooking the alkali is charged in three stages and the alkali concentration remains more eventhroughout the cook.

Oxygen delignification reduces the kappa number by about 50%. Heat and electricity demands are increased byapproximately 0.5 GJ/ADMT and 75 kWh/ADMT, respectively, but the additional recovery of organics increasesrecovery boiler heat output by roughly an equivalent amount. Heat and electricity use in bleaching may decreaseas a result of changing the bleach sequence to fewer steps, but the largest economic savings come from lowerbleaching chemicals consumption.

New batch processes were developed during the 1980s prilnarily to reduce digester energy demand since theconventional batch process uses about twice as much heat as the continuous process. It was discovered thatdisplacement heating in combination with pretreating the wood chips with weak black liquor significantly improvedpulp quality, in addition to reducing steam demand (Hakamaki and Kovasin, ]992). The basic principle is to utilizethe high grade heat in the spent pulping liquor to preheat white liquor and wood chips. (With conventional batchprocesses, digester pressure is released at the end of the cook, and the resulting flash steam is used to heat water.)

Digester primary steam consumption is typically 3.5-4.0 GJ/ADMT in the conventional batch process and 1.7-2.5GJ/ADMT in a modern batch or continuous process (Mjoberg, 1992). Energy savings ranging from 30% to 78% havebeen reported for mills with batch digesters that have converted to displacement processes (Emerfcldt and Edlund,1987; Swift and Nonnan, 1987; Tulenheilno et aI., 1992). It should be noted that the evaporator load and theprospects for heat recovery from the digester are different for different digester process designs. For example,evaporator steam demand increases by about 0.5 GJ/ADMT with the displacement batch process since water that goesoff as flash steam in the conventional batch process must now be evaporated when concentrating the black liquor.

:l.1~3& Medium Consistency ProcessingA large amount of electricity is used in pumps in the fiber line following the digester, where the pulp is washed,screened and bleached (Fig. 2.4). Typically the consistency in these operations is 1-3% in washers, 1% in screening,and 3.. 12% in bleaching. They involve several dilution and thickening loops, resulting in the pumping of largevolumes of water and high electricity demand (Simonsen and Davy, 1993). Raising consistency can reduce thevolumes of material to be pumped, but it also increa..~es friction losses in piping.

2 Theoretical energy demand for lifting 1 tonne 1 m is 9.8 kJ or 2.7 Wh/tonne.

3 Since roughly 4 tonnes of wood chips with 500/0 moisture are needed to produce one ADMT of pulp, thecorresponding electricity demand per ADMT of pulp is 2, 20, 60, and 40 Wh per meter.

4 The amount of lignin in the pulp as percent of dry pulp is roughly equivalent to the kappa number multiplied by0.15. Thus, a kappa number of 30, which is a typical target for bleached kraft pulp, corresponds to 4~5% lignin.With modified cooking processes, it is possible to reach kappa 20.

26

Environmental and energy conservation concerns are motivating the development of medium consistency (8-15%)processes (Gullichsen 1985, Tulenhcimo et at, 1992). One study estimates that mill-wide electricity demand canbe reduced by 50-75 kWh/ADMT (or about 10% of typical 1990 levels) by converting the fiber line to mediumconsistency processing. However, capital costs are too high to make it cost-effective in retrofit applications(Simonsen and Davy, 1993). Also, some unit operations are not yet fully commercial. For example, mediumconsistency pumping is now an established commercial technology in widespread use, but commercially availableequipment for screening at present is limited to 4-6% consistency. Medium consistency processing technology islikely to be installed successively when existing equipment is retired and as it is further developed to meetcommercial requirements.

For the model mill 2000 in Tables 3.1 and 3.2 it is assumed that medium consistency is used throughout the fiberline in all pumps, mixers and screens, resulting in significantly lower electricity demand for washing, screening andbleaching. In addition to reduced pumping electricity, further savings in electricity demand for pumping may bepossible through the installation of variable speed drives (see Section 3.4)..

3.1.4. BleachingBleaching technology is undergoing rapid change, and the many alternative process designs and bleaching sequencesmake it difficult to compare energy use or assess the energy impact of specific measures. The. bleaching of pulpis done in a number of stages with intermediate washing stages. Heating and cooling is needed in some cases toreach desired reaction temperatures (typically 40-90°C), whereas most of the bleach plant electricity use is forpumping and mixing. The relatively low reactor temperatures makes it possible to use mainly low grade waste heat.Electricity demand is typically 20-30 kWh per bleaching step (Jonsson et al., 1977a; Davy and Simonsen, 1993).Thus, fewer steps will result in lower electricity demand, all other things being equaL

A large amount of electricity is consumed in the manufacture of bleaching chemicals. Most of these are purchasedand therefore not accounted for in the bleach plant electricity consumption or mill energy balance. Table 3.3 showstypical chemicals consumption for a conventional chlorine/chlorine dioxide bleach sequence with and without oxygendelignification. It also shows electricity delnand for the production of the chemicals. The increase in on-site energydemand from using oxygen delignification may be less than the decrease in off-site electricity demand formanufacturing the additional bleaching chemicals that would otherwise be needed.

Environmental concern is now driving the development of elemental chlorine free (ECF) and totally chlorine free(TCF) bleaching technology. The transition is in part facilitated by extended delignification cooking, and oxygendelignification prior to the bleach plant. Ozone is currently a primary alternative bleaching chemicaL It is used insequences with chlorine dioxide in ECF, or with hydrogen peroxide in TCF bleaching. Ozone is generated on-siteand therefore shifts electricity demand from the chemicals manufacturers to the mill. Ozone consumption can be5-7 kg/ADMT or more. The electricity demand for the ozone generation (at 4% 0 3 concentration) in oxygen feedsystems is 7.5-9 kWh/kg 0 3 (38-63 kWh/ADMT).5 Thus, using ozone can result in a 5-10% increase in on-siteelectricity demand. About 85-90% of the electricity input to the ozone generator is lost as low grade heat. More islost for higher ozone concentrations (Byrd and Knoemschild, 1992).

Various studies have shown that ECF and TCF bleaching sequences have higher on-site thermal and electrical energydemands, and the total cost to build and operate a greenfield TCF mill could increase by 10% compared to aconventional mill (Davy and Simonsen 1993; Brunner and Pulliam, 1993; Mannisto et aI., 1995). However, rapidonging development of ECF and TCF bleaching tcchnology could reduce differences in total costs and energydemands.

5 Electricity demand for oxygen production would be 12.5 kWh/kg of ozone assuming a yield of 0.04 kg ozone perkg oxygen and an electricity demand of 0.5 kWh per kg oxygen. Total electricity demand under these assumptionsis, therefore, about 20 kWh/kg of alone. However, the oxygen carrier gas can be used elsewhere in the min, orrecirculated to the ozone generator.

27

3.1.5. Pulp dryingDewatering and drying of pulp in a pulp machine is normally done using silnilar principles as drying of paper in awet end, press section and a drying section (see also Sections 2.2 and 3.3). Following dewatering and pressing, thepulp can be dried as a web on cylinders, similar to paper drying, or in an air-float dryer where heated air is blownto support the web and evaporate the water (Fig. 3.1). Air-float driers are claimed to give a better quality productsince they avoid subjecting the web surface to high temperatures (perkins and Cowan, 1989). Typical web surfacetemperatures are 65-70°C compared to 90-100°C in cylinder drying. The pulp can also be dried in a flash dryerwhere fluffed pulp is introduced into a stream of hot air and then separated out in a cyclone. Most energy use ina dryer goes to evaporate water. Electricity is used in the wet end vacuum suction boxes, for machine drives, andfor fans and pumps. Typical reported steam and electricity demands in a pulp machine are 3.3-3.5 GJ/ADMT and130-150 kWh/ADMT, respectively (~onsson et aI., 1977a).

Consistency following the press section is typically 45% or less for kraft pulps but it can reach 50% or higher (whichresults in large steam savings for drying) by using state-of-the-art double wire technology (perala and Sampi, 1993).This technology also needs lower vacuum in the wet end dewatering thus reducing electricity demand. For example.electricity use decreased from 171 kWh/ADMT to 141 kWh/ADMT in one retrofit where four fourdrinier (single­wire) pulp machines were replaced with a double-wire machine. Total steam consumption in the drying line wasreduced from 1.5 tonne of steam per ADMT of pulp (3.39 GJ/ADMT) to 1 tonne of steam perADMT (2.26GJ/ADMT) (Ekebro-Graeve and Sampi, 1994).

Drying of fluffed pulp in a steam dryer (similar technology can be used for drying biomass fuels) is a potentiallyinteresting technology depending on the mill energy balance. Primary steam at 10 bar or higher is condensed onthe outside of tubes carrying the pulp wherein secondary steam at about 3 bar is generated from the water in the pulpfeed, leaving behind dried pulp (Munter, 1990; Gavclin, 1993; Gavelin, 1982). In a simplified heat balance the inputis about 3.5 GJ/ADMT of high pressure steam and the output is about 2.5 GJ/ADMT of low pressure steam and1 GJ/ADMT of clean condensate at high pressure. A drawback with this technology is the loss of relatively highpressure steam that could otherwise be used for power production.

3.1.60 Black liquor evaporationBlack liquor concentration is usually the biggest single steam using operation in a kraft pulp mill (Table 3.2). Inan evaporator, primary steam is condensed on one side of a heat-exchanger and steam is generated from the waterin the black liquor on the other side. This steam is used on the condensation side in the next evaporator effect,which operates at a lower pressure and temperature than the preceeding 'effect. To minimize primary steam use,typically 5 to 7 effects are combined in series as a multiple effect evaporator (MEE) (Fig. 3.2). The steamtemperature decreases from about 140°C in the first effect to about 60°C in the last effect. Two basic evaporatordesigns are rising film and falling film. New installations almost exclusively use the falling film design, whichoperates with a lower temperature difference across the heat exchanger surface and is able to handle higher blackliquor solids contents (Rausher, 1995).

'rypical primary steam demands in a falling-film MEE are 670 kJ per kg of water evaporated (kJ/kgw) with foureffects, 530 kJ/kgw with five effects, 440 kJ/kgw with six effects, and 390 kJ/kgw with seven effects (Jonsson et aI.,1977a; Nygaard, 1986).6 The optimal number of effects is a trade-off between operating costs and capital costs.Evaporators in the 1960s and 1970s were built with four or five effects, whereas most kraft mills today use five­or six-effect evaporators, with a concentrator to further increase solids content7 Firing the recovery boiler at highersolids content improves overall boiler performancc and is a general trend in the industry. Increasing viscosity of the

6 For comparison, another source reports that 580 kJ/kgw and 460 kJ/kgw is typical for five and six effects,respectively (Grace, 1989). One case study of a six effect evaporator installed in 1982 reports that steam use is440 kJ/kgw (Anderson and Jansson, 1983).

7 A concentrator is essentially a final evaporator stagc, but is distinguished by a different design and by the use ofprimary steam.

28

black liquor makes it difficult, though not impossible, to handle at a solids content above 75%.8

The solids content of black liquor reaching the evaporator depends on how much it is diluted in the brownstockwashing, Le., where the fibers are separated from the spent pulping chemicals and dissolved organics, and this affectssteam demand. For example, 6.2 kg of water per kg solids must be evaporated to increase solids from 13% to 65%as is done in a MEE system with six effects at the 1980 U.S. mill (Table 3.2). Solids are increased from 18% to85% in the vintage 2000 mill, requiring evaporation of only 4.4 kg of water per kg solids, or 29% less than the 1980U.S. mill. Concentrating from 13% to 18% requires evaporation of 2.14 kg water whereas going from 65% to 85%solids requires evaporation of only 0.36 kg water. The vintage 2000 mill uses a seven effect system with asuperconcentrator--an evaporator designed to handle very high solids.

Two other evaporator technologies are worth noting. A vapor compression evaporation (VeE) system upgrades thequality of steam by compressing it. In principle, steam demand can be replaced entirely by mechanical energy bytaking steam from the evaporation side of a single evaporator stage and feeding it to the condensation side (Fig. 3.3).In practice multiple VeE stages would be needed,9 and thus the use of VCE is limited to pre-evaporationapplications. Even pre-evaporation applications are limited due to the relative costs of electricity and steam at mostkraft mills. According to one source there were only six installed VeE systems in kraft mills ten years ago (Ostmanand Sebbas, 1985).

Freeze concentration of black liquor also substitutes steam for electricity. The basic principle is to subject the blackliquor to freezing temperatures. Only pure water will enter the frozen state and can be mechanically removed. Thistechnology was investigated in the 1970s and 1980s, but has never been tried in any commercial applications. Atthe time, cost assessments indicated that freeze concentration would not be as economically attractive as MEE, exceptwhere electricity prices were very low (Coleman, 1986).

3$le7~ Lime kilnThe lime mud from the causticizing reactions is washed to remove the white liquor and the percent solids in the limemud is increased to 700/0 or higher in a filter (Fig. 2.5). The lime kiln where lime mud is calcined to regenerate lime(CaO) is typically fired with oil or natural gas but it can also be fired with gasified biomass. The reaction isendothermic and requires about 3.2 GJ per tonne of CaO (Adams, 1989). The average fuel consumption in 32Canadian lime kilns surveyed in 1982 wa.(,) 10.1 GJ per tonne of kiln discharge (Simonsen and Azamiouch, 1987),equivalent to 11.2 GJ per tonne of active CaO assuming that 10% of kiln discharge is inerts. This corresponds to2.7 GJ/ADMT assuming that a charge of 240 kg active CaO is used per ADMT in the causticizer (Grace, 1989a).The average lime kiln fuel consumption in Swedish kraft pulp mills in 1988 was 1.8 GJ/ADMT (AF-IPK, 1989).10This suggests that efficiencies are higher in Swedish lime kilns but the numbers may not be strictly comparable dueto differences in alkali consumption per ADMT in the digester.ll The vintage-2000 mill (Table 3.2) assumes useof 1.3 GJ/ADMT, suggesting that more than half of the fuel input would be used for the actual calcining reaction(Warnquist, 1989). This level of fuel consumption is consistent with calculations of the efficiency improvement thatcould follow from process modifications involving e.g, higher dry solids content in the lime mud, less inerts, andlower exhaust and radiative heat losses (Theliander and Gren, 1986). Additional improvements in overall mill energy

8 Heating is used to reduce viscosity at high solids contents (Ryham, 1992).

9 As the black liquor solids content increases, higher temperatures are required to continue ev~porating \vater. This"boiling point rise" necessitates the use of several VeE stages Lo reach high solids contents.

10 Another example is an Austrian pulp mill which converted from oil firing to gasified bark and wood waste in 1986and reportedly used 2 GJ of gas per ADMT (Schweizer et aI., 1987).

11 According to a North American source (Clayton et al., 1989) the white liquor typically contains 105-155grams/liter of NaOH with a median of 122, whereas a Swedish source (Mjoberg, 1992) gives 80-100 gil of NaOHas typical values. We have not established whether the differences arc offset by a higher white liquor charge inScandinavian mills or if lhey reflect differences in wood species and pulp qualities.

29

efficiency can be achieved by, for example, pressurizing the white liquor preparation and by increasing theconcentration and causticity12 of the white liquor (Theliander and Gren, 1986; Larsson et at, 1992).

Black liquor gasification (see Section 4.2.1) may offer an opportunity to eliminate the lime cycle entirely, reducingenergy use and the cost of the recovery system. Autocausticizing refers to the direct regeneration of sodiumhydroxide (used in white liquor) from the bed solids (primarily sodium carbonate) of a gasifier in which metal oxides(e.g., Fe:z0 3' Ti02, MnOi) have participated in the gasification reactions. The metal oxide acts as a catalyst in thedirect conversion of Na2C03 to NaOH.13 Autocausticizing is similar to the direct alkaline recovery system (DARS)process using Fe:z03 for the recovery of chemicals from black liquor produced by the soda pulping process (Gleadowet al., 1993a; Scott-Young, 1995). The introduction of autocausticizing in kraft mills is expected within a decade,but it could be two decades before it is widely used (Fleischman and Sobczynski, 1993).

3.2. Newsprint from TMp14High yield pulping processes are electricity intensive. The electricity demand varies considerably depending ondesired pulp qualities and is lower for groundwood pulps than for TMP pulps (Table 3.4). One measure of pulpquality is freeness, where lower freeness means a more finely ground pulp, which in tum requires more energy.Freeness is lowest for newsprint and catalog qualities and highest for fluff. The type of wood used also affectsenergy demand, and some wood species are better suited for some processes. There has been relatively little progressin decreasing electricity demand in mechanical pulping so far. Very little heat is used in the mechanical pulpingprocesses. For example, less than 0.5 GJ/ADMT on average is used for TMP operations in Sweden (AF-IPK, 1989).

Much of the improvement in energy efficiency has resulted from increased heat recovery where the recovered steamis used to dry the paper. It is possible to recover as much as 65% of the electrical energy used in the TMP refinersas steam that can then be used for paper drying (see Section 3.3). This corresponds to about 4 GJ/ADMT. Theamount of heat recovered as steam in the T:rvtP mills that used heat recovery in Sweden in 1988 was on average 32%of total average electricity demand, or 2.7 GJ/ADMT. The level of heat recovery ranged from 18-50% in individualmills. The average steam demand in newsprint production, primarily for paper drying, was 4.9 GJ/ADMT of which2.0 GJ/ADMT was recovered steam, 1.5 GJ/ADMT caIne from purchased fuels, and 1.4 GJ/ADMT came from barkand other self generated fuels (AF-IPI(, 1989).

The minimum electricity demand, in principle, to produce mechanical pulp from logs has been estimated to be300-400 kWh/ADMT (Mohlin, 1987), which is far below actual electricity use and thus suggests that majorimprovements should be possible. It is difficult to estirnate the technical potential for energy savings in mechanicalpulping since most research results are proprietary and process changes generally also result in pulp quality changes.One energy saving measure is to switch from single-disc refiners, which use about 2200 kWh/ADMT in the refinerstage, to double-disc refiners, which use about 1800 kWh/ADMT. These numbers do not including refining of thescreening reject which requires an additional 200-300 kWh/ADMT (Wikberg, 1994). Other energy saving measuresinclude improving process control, higher refiner speed, changing the design of the grinder segments on the discs,refining at lower concentrations, and various combinations of thermal, mechanical, chemical, or enzyme pretreatment.Some sources suggest that electricity use in TMP could be reduced by about 50% (payry, 1989; Christiansson, 1995).One equipment manufacturer is reportedly undertaking full scale tests where elecLricity use is reduced by 30%,largely by changing the operating strategy (Christiansson, 1995).

For a conceptual greenfield newsprint mill design using technology that is projected to be available in the year 2000,the calculated electricitY(Cdemand is 1495 kWh/ADMT of paper made from TMP (60%) and recycled pulp (40%),Table 3.5. The Tl\1P fraction would use 1475 kWh/ADMT of virgin pulp. Pulping of the recycled fiber, including

12 The amount of NaOH divided by total titratablc alkali (11"'A = NaOH + NazS + NazC03).

13 The two basic reaction steps are: NazC03 + ~Oy -> NazMxOY+1 + COz and NaZ~OY+l + H20 -> 2NaOH + ~OY'

where ~Oy represents the metal oxide catalyst.

14 This section draws heavily from Christiansson (1995).

30

mixing the two pulps, would use 475 kWh/ADMT of recycled pulp. This advanced design newsprint mill wouldpurchase all electricity and have about 0.25 GJ/ADMT surplus steam, 0.47 GJ/ADMT bark, and 0.22 GJ/ADMTmethane from anaerobic waste water treatment that could be exported from the mill.

3.3. Pressing and drying paperWater removal from pulp slurry is among the largest steam users at any mill (see Table 3.2 for a kraft pulp mill).Typically, over 100 tonnes of water must be removed per ADMT of product. Two-thirds of the energy needed todo this is used in the drying section of a paper (or pulp) machine to remove the final 1% of this water (Table 3.6).

Paper drying starts by heating the sheet'from the temperature leaving the press section. The most common type ofdrying technology is the multiple cylinder machine, where the sheet is brought in contact with a series of steamheated cast iron cylinders (see Fig. 2.8). Increasing the consistency of the sheet leaving the press section can reduceenergy costs, but capital and O&M costs are likely to increase (Gavelin, 1982). The choice of technology in thepress section also depends on what paper quality is produced. In the dryer, most of the energy is used to evaporatewater, but a significant amount goes to other uses, especially where dryer energy efficiency is poor (Table 3.7).

A Canadian survey of newsprint dryer sections reported 36% to 45% consistency in the web after the press section,with an average of roughly 400/0 (Sayegh, et al., 1988). A Swedish survey reported a range between 39% and 47%,with an average of 43% for papers weighing between 40 and 60 g/m2, some of which are newsprint (Christerson,1993). The consistency that can be reached in modern press sections is reported to be above 50% (Hansson, 1988).Assuming that an energy demand of 2.83 MJ/kg of evaporated water (MJ/kgw) can be reached, Table 3.7, the energyneeded for drying from 50% to 90% is 2.264 GJ/ADMT of paper produced.

The reported steam consumption in the Canadian survey of newsprint dryer sections is 1.2-2.3 kg of steam per kgof water evaporated (kg/kgw). The average reported steam consumption was 1.54 kgjkgw and 2.0 kg, per kg of paper,equivalent to about 4.5 GJ/ADMT of paper (Sayegh, et aI., 1988).15 The reported steam consumption in the surveyof Swedish paper machines ranged from 2.4 to 5.5 GJ/ADMT with an average of 3.39 GJ/ADMT (Christerson,1993).

Important ways of improving the efficiency of paper drying, in addition to higher solids from the press section,include reducing overall heat losses, using less air, and reducing the amount of steam going to the condenser, Table3.7. Several technologies to increase solids from the press section and alternatives to the conventional cylinder dryingthat would impact energy use are being developed or already in use.

A common feature with new press technologies is that they operate at higher temperatures (to decrease the viscosityof the water) and higher pressures, in longer press nips, i.e., where pressure is applied to squeeze out water fromthe sheet. The extended nip press (Fig. 3.4) is a technology which was originally introduced by Beloit in 1980 andsince then other major equipment manufacturers have developed their own designs using similar principles. Morerevolutionary drying concepl~ include the Condebelt process and impulse drying.

In the condebelt process (Fig. 3.5), the web is carried between a metal and a plastic mesh, which in tum aresandwiched between two steel bands, one heated and one cooled (Lehtinen, 1992). Steam is generated from waterin the web by heat from one of the bands, and the steam condenses on the cooled band. This creates a significantpressure gradient that mechanically forces water out of the web. It is expected that most paper grades can be driedto final or near final dryness in two to three passes (Lehtinen, 1992). The process is in the pilot plant stage. Theenergy usc is expected to be some 2.2 to 3.0 MJ/kgw (Hansson,1988). The lower end of this range is some 30%below levels achieved in conventional commercial driers (see footnote 15).

With impulse drying, the wet paper web passes through a press nip in which one of the rolls is maintained at a hightemperature. In principle, the layout would be similar to the extended nip press (Fig. 3.4), but the press roll

15 1.54 kgjkgw is equivalent to 3480 kJ/kgwassuming 2.26 MJ/kgs. Another source reports that the post-retrofit steamconsumption in a newsprint paper machine is 1.25-1.34 kgjkgw equivalent to 2.8-3.0 MJ/kgw (persson, 1988).

31

temperature would be 200-4000C instead of below 10crC in a conventional press. A steam layer adjacent to theheated surface grows and displaces water from the sheet without evaporating it. The energy demand can be half orless compared to conventional evaporative drying (Lavery, 1988). Size and capital costs are likely to be lower sinceimpulse drying has water removal rates16 that are 100-1000 times those found in conventional drying sections. Apotential application of impulse drying is to increase solids up to 60-70% or higher, followed by evaporative drying,which would then require many fewer cylinders and significantly reduced energy inputs compared to conventionaldrying. Total drying energy use has been estimated to be 0.6-1.4 MJ/kgw (Hansson, 1988), or 50-80% less thanconventional drying. Research in recent years on impluse drying has focused on reducing delamination of the sheet,which has been a particular problem with thick paper grades (Orloff, 1991).

3.4. Technologies used throughout the industryElectricity end-uses common to all pulp and paper mills include pumping, air-handling, and lighting. In addition,steam "needs and the large number of process streams makes the industry a good candidate for improved heatintegration. Information technology, e.g., sensors, computers and controls systems, is developing rapidly and offersthe potential for higher product quality and lower energy use. Other generic energy uses that will not be discussedhere include compressed air systems, mixers, paper machine drives, vacuum pumps, and energy use in offices.

In addition to taking measures to reduce fluid flows, energy use in pumping and air-handling can be reduced byimproving pump, fan, and other component efficiencies, reducing flow resistances, and by applying variable speedcontrol instead of throttle control. Pumps and fans typically account for 40-45% and 15-20% of total electricity usein kraft pulp mills (Larson and Nilsson, 1991).

An energy analysis of one kraft pulp mill in the U.S. shows power demand to be nearly constant over a range ofproduction rates (Fig. 3.6). This indicates that pumps and fans are operated with throttle control (power demand inpumps and fans at reduced flow is essentially the same as power demand at full flow when throttle control is used).Power demand would decrease with reduced flow if variable speed control were to be used.17

Average potential cost-effective savings through replacing worn pumps, downsizing oversized equipment, installingvariable speed drives, etc., for pumps greater than 50 kW have been estimated based on a number of audits inScandinavian pulp and paper mills to be about 300/0 (IndMeas, 1989). Similar results are reported for some U.S.mills (Herzog et aI., 1992). The results of a pumping system audit for a specific pulp mill are shown in Table 3.8.Most of the savings would be achieved by replacing worn or oversized pumps and motors, or by trimming pumpimpellers. Although throttling losses associated with oversized equipment operating at more-or-Iess constant loadscan be reduced by applying variable speed control, it will often be less expensive to downsize the equipment thanto invest in a frequency converter for speed control. Another interesting example is the application of speed controlto medium consistency pumps. This technology wa.~ first tried in a bleach plant retrofit in 1990 at a paperboard millin Sweden. Pumping electricity use decreased by 26-27%, or 20 kWh/ADMT, and the payback period was two years(Caddet, 1993). Throttling valves are still needed at very low flows because medium-consistency pumps mustmaintain a specified minimum speed to insure that the fiber suspension remains fluidized.

Air-handling systems with variable flow are also good candidates for variable speed control. For example, one casestudy estimated that replacing variable inlet vane control of boiler fans with variable speed drives would result incost-effective savings of 50% (Nyberg, 1989b). Natural ventilation driven by temperature differences can replaceconventional mixing ventilation to supply fresh air in some cases. One paper mill recently eliminated 160 kWofinstalled fan power Lhat was used to blow fresh air from the ceiling to the work zone in a factory building (Burangen,1990). Enough heat is generated by the equipment to drive the circulation of air in a natural draft ventilation systemwith the· air-intakes placed at the work zone level (Fig. 3.7). Waste heat is used to preheat the incoming air tocomfortable levels.

16 Kg water removed per m2 and hour4

17 The savings are possible because the input power to many pumps and fans varies with the cube of flow, while flowvaries directly with speed.

32

High quality illumination is important for safely and to enhance productivity and working conditions. Lightingtypically accounts for only a few percent of mill electricity demand, but this corresponds to several hundred kilowattsof electric load. Modern lighting systems based on high pressure sodium lamps can reduce electricity costs by 50­80% relative to systems with mercury vapor lamps. There would be similar savings in maintenance costs, and thepay-back in typical retrofit applications ranges fTom 5 to 22 months (Maillet AJ., 1985).

Heat and power integration, Le., the integration of heal-exchangers, heat pumps, heat-recovery systems, combinedheat and power generation, etc., is critical for energy efficient operation of a mill. Pinch analysis is a systematicmethodology that can be used to optimize this integration. Reported energy savings range from 10-35% with simplepayback times of 1-3 years for pulp and paper mill retrofil~ made based on pinch analyses (Karp, 1990). In anothercase, pinch analysis identified cost effective savings corresponding to 9% of the heat demand in an already energyefficient kraft pulp mill that used 15-16 GJ/ADMT (persson et aI., 1990).

It appears that·the pulp and paper industry has been slow to adopt new tcchnologies for process control, analysis,and automation (Stephens, 1992; Bialkowski, 1991). Energy efficiency in the pulp and paper industry is affectedin several ways by information technology. Computer programs aid the designer of plants and processes. On-linesensors and process control systems can provide precise control of anything from an individual pump to a papermachine. Computers help operators to monitor and optimize the operation of unit processes and whole plants.Specific to the pulp and paper industry, the development of on-line sensors for a number of difficult to measurepaper, pulp, and wood properties is important. It is difficult to assess how computer control compared to manualcontrol could impact energy use specifically since new control systems are often implemented in combination withother process changes and involve considerable operator training. We have not found any reported results from thepulp and paper industry but a detailed evaluation of a control system for steel reheating furnaces in the steel industrymay serve as an illustration. Here it was concluded that computer control may well result in 15-20% savings, largelydue to the information handling capability, speed and precision of computers (MArtensson, 1994).

3..5 Energy Impacts of Environmentally Driven TechnologyEfforts to reduce emissions from pulp and paper mills includc external control measures, e.g. waste water treatmentplants and electrostatic precipitators, as well as internal process changes, e.g., oxygen delignification with return ofmore organics to the recovery boiler. External control measurcs, for example, an aerated lagoon waste watertreatment plant, often increase energy demand. However, the total energy impact depends on technology choice.For example, the anaerobic treaunent of papennill sludges in model mill 2000 in Table 3.5. requires 85 kWh/ADMT.This is partly off-set by the heating value of the generated methane, 0.22 GJ/ADMT (Warnquist, 1989). Electricityuse in an electrostatic precipitator corresponds to about 6 kWh/ADMT, but it allows the recovery and reuse ofsodium and sulfur from the precipitator catch (J6nsson et aI., 1977a). Oxygen deIignification can result in increasedon-site energy demand, which is partly offset by burning more organics in the recovery boiler, but if the lower useof bleaching chemicals is accounted for (see Section 3.1.4), it may lead to an overall decrease in energy use.

Complete elimination of solid waste and of all emissions to air and water from pulp and paper mills is not possible.The industry is pursuing the concept of "closed cycle" operation, particularly as it relates to the bleaching cycle tominimize liquid and gaseous emissions. One important characteristic of mill closure will be the return to therecovery boiler of essentially aU organic material and reaction products dissolved in post-delignification and post­bleaching wash stages. Complete closed-cycle operation is not possible, because minerals, metals, and other "non­process elements" that enter the mill with the wood or process chemicals must be removed to prevent theircontinuous buildup. Some key non-process elements are chloride, potassium, phosphorous, calcium, silicon, and anumber of metals. Some non-process elements will havc natural points in the process from where they can bepurged. Others may be more problematic. In any casc, closed cycle operation will mean that emissions of solidwaste win take on greater importance. Closed cycle operation may also require more de-coupling of process areasthrough buffer storage capacity. Advanced process control and monitoring systems will also likely be necessary to

effectively deal with disturbances, because the margin for deviations from design operating conditions are likely todecrease.

Some modeling studies suggest that a closed cycle bleached kraft pulp mill may have 10-20% higher steam andelectricity demands than a conventional min due the need Lo concentrate bleach plant effluents, waste water treaunent,

33

and the addition of new processes and equipment (Glcadow et aI., 1993b; Patrick et al., 1994). However, some 4-8%more steam can be generated in the recovery boiler duc to the relum of more organics (Mannisto et at, 1995; Patricket al., 1994; Gleadow et aI., 1993b).

34

Table 3.1. Comparison of specific energy use in kraft pulp mills

Including powerhousell Excluding powerhouses

Steam Electricity Steam Electricity Electricity-demand demand demand demand to-heat

Mill (GJ/ADMT) (kWh/ADMn (GJ/ADMT) (kWh/ADMn (kWh/GJ)

1960 U.S. millb 24.5 920 22.3 756 34

1980 U.S. mille 20.2 780 16.3 656 40

Average 1988 Swedishd 15.2 840 - .. 55d

Best 1988 Swedishd 12.4 720 .. - 58d

Model mill 198(f 12.2 740 11.7 680 58

Model mill 2000f 7.8 640 7.8 580 84

(a) Powerhouse energy demand is the cogeneration plant's parasitic energy consumption.

(b) Source: (Subbiah et at 1995)

(c) Source: (Larson, 1992).

(d) Steam data for average and best 1988 Swedish mills (AF-IPK, 1989). Swedish data were collected by AF-IPKon behalf of the energy committee of the Swedish Forestry Industries Association.

(e) Electricity to heat ratio is based on steam and electricity demand including powerhouse because separate estimatesof powerhouse steam and elcctricity consumption were not available.

(0 Data for the model mill 2000 is from (Wamquist, 1989). The model mills shows what is considered to betechnically and economically feasible in a greenfield mill with 1980 and 2000-vintage technology, respectively. Themodel mill 2000 is still based on the kraft process but uses extended modified cooking, oxygen delignification andelemental chlorine free bleaching.

35

Table 3.20 Comparison of unit-level steam and electricity demands fOf bleached kraft pulp production in a U.S. millbuilt in the 1960s, US60 (Subbiah et at, 1995), a U.S. mill built in the 1980s, US80, adopted from (Larson, 1992),a hypothetical mill that would use the best available technology in 1980, M80, and in 2000, MOO, both adopted from(Warnquist, 1989). Note that different accounting principles complicates comparisons at the unit level.-

I II STEAM (GJ/ADMT) I ELECTRICITY (kWh/ADMT) IIMill identifier ~ I US60 US80 M80 MOO US60 US80 M80 MOO

Woodyard n.a. n.a. 0.3 0.1 - 25 75 75

Digester 4.57 2.89 2.5 1.5 43 50 40

Washing n.a. n.a. 0 0 40 10

Screening n.a. n.a. 0 0 212 103 45 20

Oxygen delig. 0.43 n.a. 0.35 0.7 47 60 85

Bleachingb 1.15 0.5] 1.35 0.15 185 42 120 60

Screening andstorage n.a. 1.08 0 0.2 o.a. 74 50 40

Drying and baling 5.92 3.94 3.25 2.15 174 153 130 110

Black liquorconcentration 5.26 4.33 3.75 2.95 n.a. 66 25 30

Powerhouse 2.50c 3.91 0.50 0 n.a. 125 60 60

Caust. & limekiln (exc!. fuel) 0.31 n.a. 0.15 0 141 42 35 45

Waste water n.a. n.a. 0 0 n.a. 35 30treaunent

Other 4.41 d 3.51c 0 - 20gf 61 8 15 35

ITOTAL 11 24.55 1 20.16 I 12.15 I 7.75 I 920 I 780 I 740 I 640 I(a) For example, the high electricity use in causticizing and lime kiln in the U560 min suggests that it includeselectricity use in black liquor evaporation.

(b) The US80 mill has a 3-stcp bleaching sequence and the other mills have 5-steps bleaching.

(c) Consists of 1.44 GJ/ADMT and 1.06 GJ/ADMT to utilities and soot blowing, respectively.

(d) Consists of 2.34 GJ/ADMT, 1.01 GJ/ADMT, and 1.06 GJ/ADMT to deaeratof, water heater, and chiller.

(e) Consists of 1.75 GJ/ADMT, 0.70 GJ/ADMT, and 1.06 GJ/ADMT to deaerator, chiller, and other, respectively.

(0 Consists of 164 kWh/ADMT and 44 kWh/ADMT to utilities and water plant, respectively.

(g) Consists of 16 kWh/ADMT, 25 kWh/ADMT, 5 kWh/ADMT, 2 kWh/ADMT, and 14 kWh/ADMT to watersupply, air supply, chiller/HVAC, odor control, and miscellaneous, respectively.

36

Table 3.3. Bleaching chemicals consumption for bleached kraft pulp and electricity use in their manufacture(Nygaard, 1986).

Bleach sequence ~ CEDED O-CoEDED

Chemical kWh/kg kg/ADMT kWh/ADMT kg/ADMT kWh/ADMT

Oxygen (OJll 0.5 - - 20 10

Chlorine (CIJ 1.7 70 119 35 60

Sodium hydroxide (NaOH) 1.7 46 78 20 34c

Chlorine dioxide (CIOJ 5.0 22 110 25 125

Hypochlorite (NaOCI) 3.4 12 41 - -Total 348 229

(a) Oxygen can be purchased or. made on-site. The oxygen delignification stage involves heating, pumping andwashing which increases heat and electricity demand at the mill by approximately 0.5 GJ/ADMT and75 kWh/ADMT, respectively.

(b) Chlorine dioxide is generally made on-site by mixing purchased sodium chlorate, some reducing agent, and acid.Because it decomposes explosively in the gas phase, the CI02 and is dissolved in water soon after it is formed. Theelectricity demand shown here is for the manufacturing of the purchased chemicals. Very little energy is requiredon-site.

c) The use of NaOH is lower with this bleaching sequence, despite the added use of an oxygen stage, because it hasbeen assumed that some oxidized white liquor would be used in the oxygen stage instead of NaOH, and also lessNaOH is required in the caustic extraction stages.

37

Table 3.4. Electricity use in mechanical pulping processes.

Source Application/Quality Electricity use, kWh/ADMT

Stone Groundwood and Pressurized Stone Groundwood Pulping

Swedish average 1920

newsprint 1790-2300

AF-IPK, 1989acatalog/jownal 2100-2350

board/carton 800-1290

Eliah and Lowitt, 1988b SGW typical 1260-1450

Thermomechanical pulping

Sweden average 2300

newsprint 2200-2450

AF-IPK, 1989 catalog/journal 2080

board/carton 1670-2170

Eliah and Lowitt, 1988b average TMP 1800

Wikberg, 1994 newsprint 1850-2500

Chemi-thermomechanical pulping

fluff pulp 900

Wikberg, 1994 tissue 1400-1600

printing and writing 2200-2400

Recycled paper

liner 260

AF-IPK, 1989 newsprint 370

other 440

(a) The Swedish SOW data may not be representative since they are based on a production of only 497,000 tons in1988 in much of which is probably old equipment since SGW is being replaced by TMP in Sweden. The data fromAF-IPK are the reported results of an extensive survey of energy use in the Swedish pulp and paper industry in 1988,reported electricity use is for pulp making and includes only refining, screening, and reject refining.

(b) Includes only the TMP refiner SGW itself and apparently not reject refining.

38

Table 3.5. Process heat and electricity demand per ADMT of paper produced in two hypothetical newsprint mills.Source: Warnquist, 1989.

Newsprint mill model 1980-858 Newsprint mill model 2000b

GJ/ADMT kWh/ADMT GJ/ADMT kWh/ADMT

wood preparation - 23 - 45

T:MP pulp - 1624 - 885

recycled fiber - 80 - 160

stock preparation - 75 - 30

paper machine 3.36 265 2.33 290

waste water - 34 - 85

Total 3.36 2,101 2.33 1,495

(a) Newsprint made from 80% TMP pulp and 20% recycled fiber. The refiner uses 1,750 kWh/ADMT pulp,screening and reject refining uses 280 kWh/ADMT pulp.

(b) Newsprint made from 60% TMP pulp and 40% recycled fiber. The refiner including reject refining uses1,400 kWh/ADMT pulp and screening uses 75 kWh/ADMT pulp.

39

Table 3.6. Newsprint machine water removal and energy use, adopted from (JOnsson et al., 1977b)

wet end press section drying section

percent solids, in-out 0.8-20% 20-42% 42-90%

ton water removed per ADMT 109 2.36 1.14

energy use, MJ/ADM'I' 810 972 3,824

energy use, MJ per ton water 7.44 412 3,350

(a) Electricity has been converted to primary energy using a heat rate of 10.8 MJ per kWh. Wet end and presssection use no steam. Electricity use in the drying section is 43 kWh/ADMT and steam use is 3.36 GJ/ADMT

40

Table 3.7. Calculated energy demand in kJ per kg of water for drying paper. Source: Gavelin, 1982.

Evaporated water, kJ/kg Best practice Poor practice

Heating and evap. of water' 2460 2460

Heating of fiberb 70 100

Desorption heatC 20 60

Air preheatint 140 1600

Condenser losse~ 70 300

Heat lossesf 70 300

I Total I 2830 I 4820 I(a) Assuming water is heated from 50°C and the heat of evaporation is 2.26 MJ/kg of water.

(b) Heat needed for heating the fiber from 50°C.

(c) Water binds chemically to the fibers and above 70% consistency, requiring desorption heat.

(d) Preheating 7.25 ton of dry air per ton of water assuming heat recovery.

(e) Steam condensed in the condenser.

(f) Radiative heat losses from the dryer hood.

41

Table 3.8. Electricity savings potential estimated for 12 pumps based on measurements at a pulp and paper mill inWargoen, Sweden, based on (Nyberg, 1989a) as presented in (Larson and Nilsson, 1991).

CSEC (c/kWh)Pump application and Recommended investmentsB Investment Annual savings for disc. rate =capacity (kW) M P TI VSD cost (k$)b (MWh/yr) 6% 20%

Raw-water intake (119) X X 44.3 475 (50%) 1.3 2.2

Wash water (380) X X 86.2 550 (18%) 2.1 3.6

Tank drainage (69) X 29.2 216 (42%) 1.8 3.2

Bleached pulp (36) X 8.9 43 (31%) 2.8 4.9

Return water (101) X X X 85.8 610 (68%) 1.9 3.4

Warm water filter (54) X X 19.1 192 (40%) 1.4 2.4

Mixing (184) X X 32.3 331 (23%) 1.3 203

Fresh water supply (59) X X 14.9 414 (82%) 0.5 0.9

Effluent (28) X X 13.1 130 (57%) 1.4 2.4

Paper mach. water (28) X 13.4 162 (79%) 1.1 2.0

Mixing tank (26) X 13.4 153 (78%) 1.2 2.1

Waste water (52) X 5.2 200 (48%) 0.4 0.6

ITOTALS I 6 I~ 365.8 I 3476 (38%)d I 1.4 I 2.5 I(a) M =replace motor, P =replace pump, TI = trim pump impeller, VSD =install variable-speed drive.

(b) Costs are given in thousand 1990$. Original costs in Swedish kronor (SEK) were converted using 6.5 SEK perU.S. dollar. The present exchange rate is approximately 7.5 SEK per U.S. dollar.

(c) Cost of saved electricity (CSE) calculated assuming IO-year lifetime. CSE is calculated. as the annualized capitalcosts divided by the annual electricity savings.

(d) The savings represent 26% of the electricity used by a total of 32 pumps that were originally selected for thisstudy. Detailed savings estimates were made only for those listed in this table.

42

,-- I@ ..-

'\ 1(- ,-.,y '<....-/ .-..,,--.. ~

@ @ @ @ ~ I@lilt ::::: ~ ;=.: i-- = -- '\ @ @ @y -..I@~ 1@!

- -

/@I-

@ I@ @ (@J

~i~ ~ .Ie-

~ I~'I ---- - -

Fig~ 3;J~ An air float dryer (Perkins and Cowan, 1989). The pulp web moves over the dryer decks where hot airis blown to support the web and evaporate water. Turning rolls at the end sections move the web.

43

44

,..-------------,•,ID

II

•Stea!".. l

I

•IIII,,I

r"Steamcondensate

Fig~ 3~2 .. Sample multiple effect evaporator sequence (Grace, 1989b).

Feedliquor

Entrainmentseparator

NC gas ventIilIli

Heatingsurface Compressor

makeup steam

Condensate

Product

Circulatingpump

Feed

Heat exchange

3..3" Illustration of vapor compression evaporation principle (Grace, 1989b).

45

46

CONTROl-LIEDCROWN "O&.'"

IEXTINDflD NIPIBIlEA.UNG IU"ANtClIi.,.

3"4,, Principle of the extended nip press (deBeer et aI., 1993).

/

Fine metal wire

Thick plastic wire

Fig$ 305.. Principle of the Condebelt drier (deBeer et at, 1993).

47

7

6-c~~

5~c.9a

4 §CIlCo(.,)

-"'~"·~~~""""'...!l1/'lJ"·_···, ...... •.....•..·... •......... .,·· .. •........ ·· .........~ ..... ·· .. -t-3 .~(.,)

'.5(.,)

................. "' R~!~ ..~~ ·;. · , 2 13bleach plant

O~05 ··"' "'·"" ·..,,, "' .. "" .., """' .,, "' .,. "' "" .,."..,,,,,·<0 • 1» .. .,. 1

36 38 40 42production rate (ADST/hr)

44

left scale

46°

Fig.. 3~6 .. Power demand in digesters, bleach plant, and pulp machine for a U.S. kraft pulp mill (Subbiah et aL, 1995).Power demand in digesters is dominated by pumps, in the bleach plant by pumps and mixers, and in the pulpmachine by fans and vacuum pumps.

48

f\I

i\!

1 t

ii1

1'0

I PAPER~1ACHINE

~

~

./ /' /~/ '//' :/ /'

Fig.. 3.. 741 Natural draft ventilation principle used in the paper machine area of a mill.

49

4. BIOMASS BASED COMBINED HEAT AND POWER GENERATIONIn addition to being the feedstock for pulp and paper production, biomass is also a major energy resource for theindustry. Black liquor and solid-biomass residues (bark and hog fuel) generated at the mill and used for energyamounted to some 1.5 FJ in 1993 (Table 1.3). The industry also has access to residues of pulpwood harvesting,some of which can be removed from the forest without serious environmental consequences.l One estimate of theamount of environmentally recoverable forest residues (applicable in the Southeastern U.S.) is some 0.15 tonnes pertonne of pulpwood (Larson, 1992). If applied to the total U.S. pulpwood consumption,2 this corresponds to about17 million dry tonnes of biomass, or an additional 0.34 EJ per year of biomass.

All black liquor and most mill residues are used at mill sites to fuel cogeneration systems providing stearn andelectricity for on-site use. In 1991, the industry generated a total of 54 TWh of electricity, which met 56% of itselectricity needs (MECS, 1994). Essentially all of this power was produced in steam-rankine cycles, with blackliquor accounting for about 43% and bark and wood chips for about 18% of the fuel input to the boilers (Giraldoand Hyman, 1993; MECS, 1994). The remaining 39% came from fossil fuels, predominantly natural gas, coal, andresidual fuel oil.

Energy implications of alternative biomass-fueled cogeneration technologies fuels for the industry are discussed here.

4.1. Existing cogeneration technologyThe average heating value of spent pulping liquors in the U.S. is equivalent to 23 GJ/ADMT of chemical pulp andit accounts for 73% of the biomass derived fuels used in the pulp and paper industry today.3 The concentrated blackliquor in the kraft pulping process today is burned in Tomlinson recovery boilers.. to (i) recover process chemicalsin the fonn of N~S and Na2C03 and (ii) generate steam from combustion of the contained organic matter. Therecovery boiler is thus an integral part of the process and of the mill's steam and power system. The cost of arecovery boiler for a 1,200 ADMT per day bleached softwood kraft pulp mill firing 2,000 tonnes of black liquor perday is about $65 million (McKeough et al., 1995). An entire chemical recovery system costs about $100 million,about one-sixth of the total cost for a 1,200 ADMT per day bleached kraft market pulp mill (Weyerhaeuser, 1992).The recovery boiler is a primary reason for the capital intensity and economies of scale associated with kraft pulpmills.

In a recovery boiler, droplets in a black liquor spray dry, pyrolyze and burn. The inorganic chemicals are extractedas a smelt at the bottom of the boiler. Steam is usually produced at about 60 bar and 450°C and fed to one or moreback-pressure steam turbines from which some process steam is extracted at 10-12 bar and the rest exhausts at4-5 bars" Bark, hogged fuel, and other fuels are fired in separate boilers to raise stearn which typically augmentsthe steam from the recovery boiler.

The electricity-to-heat production ratio for a conventional back-pressure steam turbine is about 0.23 (60 kWh/GJ)with no intermediate extraction and about 0.15 (40 kWh/GJ) with an intermediate extraction (Warnquist, 1992).These electricity-to-heat ratios are relatively well matched to the steam and electricity needs at older kraft mills(Table 3.1). However, increasing electricity-to-heat demand ratios (see Section 1.2) are motivating interest in

1 The impact on soil nutrients and organic content are key concerns associated with removal of logging residues.Recycling of ash from the min back to the forest may be necessary or desirable to minimize long-term nutrientdepIction (Ericsson and Borjesson 1991; Abyhammar et aI., 1994). Removing residues for energy without recyclemight actually be beneficial in areas with high nitrogen deposition from air-pollution (Lundborg, 1993).

2 Pulpwood consumption in 1993 was 67 million cords (1 cord = 128 fe) of softwood and 39 million cords ofhardwood (AFPA, 1994a), or about 110 million dry tonnes assuming a dry maller content of 2000 lb/cord (908kg/cord) for softwood and 2800 lb/cord (1,270 kg/cord) for hardwood (Clayton et aI., 1989).

3 Based on a production of 46.55 million ADMT of kraft pulp and 1.29 million ADMT of sulfite pulp (AFPA,1994a) and a total of 1.11 EJ in spent pulping liquors (Table 1.3)"

50

alternative cogeneration technologies.

Electricity-to-heat ratios can be increased to some extent by increasing boiler pressures and temperatures. Forexample, 60 to 80 kWh/GJ can be achieved with steam conditions of 105 bar and 520°C (Warnquist, 1992). Theoverall ratio of electricity to heat output may also be increased by using a condensing extraction steam turbine wheresome of the steam is expanded to sub-atmospheric pressure to produce additional power and then condensed.Condensing power can also be produced by adding a separate condensing steam turbine after the back-pressureturbine. The technology of choice depends on the specific energy balance and relative fuel and electricity prices foreach milL

There are several reasons for industry interest in alternatives to the Tomlinson recovery boiler/steam turbine systems,in addition to limitations on the electricity to heat ratios that can be achieved. The black liquor handling capacityof the recovery boiler is typically the bottleneck to expanding overall production capacity at a mill, and addingincremental recovery boiler capacity (as many mills would like to do) is prohibitively capital intensive. (Recoveryboilers are built large to take advantage of economies of scale.) Other drawbacks of Tomlinson recovery boilersinclude dangerous explosions that can occur when water or wet black liquor inadvertently contacts the smelt, as wellas emissions of odorous and acid gases. Furthermore, a large fraction of the existing recovery boilers were installedfrom 1965-1975 (Fig. 4.1). It is expected that 70% of all boilers will need some type of major rebuild orreplacement in the next decade (Patrick et aI., 1994).

4029 Future cogeneration technologyThe focus of most interest in new biomass cogeneration tcchnologies are those that would utilize gas turbines ratherthan steam turbines. Gas turbines are generally characterized by higher electricity-to-heat ratios than steam turbines,as well as lower unit capital costs. The key requirement for using biomass in a gas turbine is a clean gaseous fuel.Thus, there is significant development ongoing of technologies for converting black liquor or biomass residues intocombustible fuel gas,4 and the cleanup systems that would be needed to enable use of the gas in gas turbine cycles.

~t2019 Black liquor gasificationBlack liquor gasification (Ihren, 1994; Grace and Thnrner, 1995) is auracting considerable interest from industrytoday primarily because gasifiers are expected to be able 1.0 cost-effectively add incremental chemicals recoverycapacity to mills. In the longer term, full scale black liquor gasification/gas turbine cogeneration systems asreplacements for recovery boiler/steam turbine systems should offer higher overall energy efficiency, higherelectricity-to-heat ratios, and lower emissions than recovery boilers.

Gasification processes are categorized as low-temperature/solid-phase (below approximately 750°C) or high­temperature/smelt-phase (above approximately 9000C). They produce fuel gas with higher heating values (HHV)of 3-4 MJ/Nm3 (with air as the gasifying agent) or 8-9 MJ/Nm3 (with oxygen or with indirect heating) (Grace andTimmer, 1995). With either gasifier, cleanup of the fuel gas is required to recover inorganic chemicals that mayleave the gasifier with the gas, to prevent damage to the turbine, and to meet emissions regulations.

The main feature of the low temperature gasification processes are that they employ fluidized bed gasification. Solidsodium carbonate (Na2C03) is precipitated out during gasification and forms the bed materiaL Some 70% or moreof the sulfur in the black liquor leaves as H2S gas, which then is scrubbed from the gas (Fig. 4.2). Manufacturingand Technology Conversion International (MTCI) and Asca Brown Boveri (ABB) are two leading developers of low­temperature gasifiers. The MTCI design is an indirectly-heated fluidized bed (with in-bed heater tubes). The ABBdesign (Fig. 4.2) is an air-blown circulating fluidized bed. MTCI is testing a 50 tonne black liquor solids per daygasifier at Weyerhaeuser's New Bern kraft pulp mill in North Carolina. ABB has a tested a 2-4 tonne per day pilotgasifier in Sweden since 1991 and is planning a 50-100 tonne per day demonstration unit in the U.S.

One of the main difficulties associated with low temperature processes is that they must operate within a narrow

4 Hydrogen and carbon Inonoxide are the most itnportant combustible components in most fuel gases from gasifier.The full composition of the gas depends on the design and operating conditions of the specific gasifiers.

51

temperature window--high enough to achieve satisfactory gasification rates and tar destruction, but low enough toavoid softening of the bed material leading to agglomeration. Closed cycle operation of mills (see Section 3.5) couldlead to accumulation of potassium and chloride in the bed ma~rial. If not removed, these elements will lower themelting point of the bed and further narrow the temperature window.

High temperature gasification uses an entrained flow reactor (Fig. 4.3). Inorganic chemicals (Na2C03 and Na2S) arerecovered as a smelt similar to that from a recovery boiler. About half the sulfur leaves as H2S in the fuel gas. Thehigh reactor temperatures give higher rates of carbon conversion than with the low temperature design. KvaemerPulping is currently the leading developer of the high temperature technology. An atmospheric-pressure unit havinga capacity of 75 tonnes per day is operating commercially at a Swedish mill in parallel with a recovery boiler toboost chemicals recovery capacity. Kvaerner has also operated a pressurized (6-7 tonne per day capacity) gasifierwith a gas clean-up system at a Scandinavian mill since February 1994 (Stigsson, 1994). Large atmospheric-pressureunits--250 tonne per day--are now being offered on commercial terms for incremental capacity increases. Tampella,a Finnish equipment supplier to the pulp and paper industry, undertook limited pilot scale tests beginning in 1991with a 2-3 tonne per day gasifier. However, the unit is no longer being operated, and Tampella's activities are atpresent limited to fundamental studies (Janka, 1995).

The higher rate of carbon conversion is an important advantage of the high temperature gasifiers. There is someconcern regarding the materials and corrosion problems associated with operating at high temperatures. Higherconcentrations of potassium and chloride as a result of mill closure are likely to be an advantage in high temperaturegasifiers since they increase the reactivity of the black liquore (Salmenoja et aI., 1993: Stigsson, 1994.).

4~2~2$ Bark and wood' waste gasificationBlack liquor is the most abundant biomass energy source at a kraft pulp mill, providing some 20-25 GJ of energyper ADMT.s However, bark and wood wastes also make important contributions at most mills. Typically, 5-10%of the pulpwood that enters a mill is·bark and other residues, corresponding to roughly 200 kg of dry substance (DS)or 4 GJ per ADMT of pulp. The recovery of logging residues (see introduction to Section 4) might add another 8GJ/ADMT so that bark and wood waste might make up 12 GJ/ADMT, or 50% of the energy value of the blackliquor.

A handful of large (>30 MW biomass input) atmospheric pressure, air-blown wood-chip gasifiers are operatingcommercially today, most of which are supplying fuel gas to lime kilns at kraft pulp mills. Pressurized and otheradvanced gasifier designs are the focus of demonstration efforts of biomass integrated gasifier/gas turbine (BIG/GT)technology in several countries (Williams and Larson, 1995). The first BIG/GT system to be built is a 6 MWc and9 MWth (20 MW biomass input) combined cycle district healing cogeneration facility in Vamamo, Sweden. Testingof that system began in 1994 and is ongoing. Other planned demonstration efforts include a commercial scalecombined cycle in Northeast Brazil that will be partially financed by the Global Environmental Facility (Elliott and~ooth, 1993). Several demonstration projects in the 8-20 MWe range have recently been announced in the EuropeanUnion and two projects receiving partial backing from the Department of Energy are ongoing in the U.S.

4$2&3& Electricity exports from kraft pulp millsMills with kraft pulp production might become significant electricity exporters if they adopt full-scale black liquorand biomass gasification gas turbine technologies (Larson, 1992; Ihren, 1994; BergHn, 1995; McKeough et aI., 1995).Commercial biomass gasifier/gas turbine systems might be commercially available as soon as the end of the decade.Commercialization of systems using black liquor gasification will probably require somewhat longer, becausegasifier/gas clean up technology is at a less mature stage for black liquor than for woody biomasse

The potential impact of alternatives to existing cogeneration systems using back pressure steam turbines can beappreciated from an examination of three configurations of black liquor/biomass cogeneration systems at a kraft pulpmill, assuming biomass fuel availability of 21 GJ/ADMT of black liquor and 4 GJ/ADMT of bark: (1) biomass and

5 With closed. cycle operation, the energy available from black liquor would be somewhat higher than this.

52

recovery boilers are used, but a condensing-extraction steam turbine (CEST) replaces the back-pressure system; (2)black liquor and biomass gasifiers fuel a gas turbine/steam turbine combined cycle; and (3) the same as the previoussystem, but assuming the supplemental availability of 8 GJ/ADMT of logging residues as fuel.

Figure 4.4 from Subbiah, et al (1995) summarizes the estimates of electricity and steam production for each case.Varying electricity-to-heat ratios can be generated with the CEST and gas turbine technologies to match the particularenergy demands of a mill. Mill demands correspond to those shown in Table 3.1 for alternative mills.

The CEST option in the full cogeneration mode (maximum process steam production) generates about 600kWh/ADMT of electricity and 14.4 GJ/ADMT of steam. With this tcchnology an efficient kraft mill can beessentially energy self-sufficient. With option 2--Lhe integrated gasification/gas turbine combined cycle using millresidues--l,800 kWh/ADMT and 11 GJ/ADMT would be generated in the full cogeneration mode. Further reductionsin steam demand would be needed compared to option 1. Assuming these can be achieved, there would be about1100 kWh/ADMT, or 35 to 45 MW of baseload power (at a 1200 ADMT/day mill), available for export from themill after meeting on-site demands. The value of the excess electricity (assuming 5 c/kWh revenue) would be$55/ADMT, which is significant relative to the value of the primary product, pulp. (pulp prices range from $400 to$800 per ADMT.) The same system but including use of logging residues (Option 3) would generate2440 kWh/ADMT and 14.5 GJ/ADMT in the full cogeneration mode, and would thus provide still greater electricityrevenues. The economic viability of the gas turbine options would need to be examined carefully, but somepreliminary assessments (McKeough et ai, 1995; Larson, 1992) show potentially attractive returns.

Assuming an average electricity production of 2000 kWh/ADMT of bleached, semi-bleached, and unbleached kraftpulp, and a production of 50 million ADMT of kraft pulp per year, the total annual electricity production in U.S. .kraft pulp mills could be 100 TWh. This is roughly double the amount of electricity self-generated by the pulp andpaper industry today. It is the equivalent of total electricity consumed by all pulp, paper, and paperboard mills inthe U.S.

53

25 ....,.....---------------------------------

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54

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19401945195019551960196519701975198019851990

Start-up year

~tl .. Number of recovery boilers by start-up year or latest rebuild year. Adapted from Patrick, et al. (1994).

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tt2'8 Lowotemperature/solidQphase, circulating fluidized bed black liquor gasifier (ABB type) with wet gas cleanup (Ihren, 1994).

55

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430 High...temperature/smelt-phase, entrained-bed black liquor gasifier with quench (Kvaemer type) and wet gasclean up (Ihren, 1994).. Pressurization of the gasifier facilitates recovery of heat at higher temperature and pressure.

56

Gasifier/combined cycle + wood residues

Gasifier/combined cycle

25

1960 U.S.

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1 0

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Fig.. 4..4.. Stearn and electricity production potential (net of cogeration plant) at ,a kraft pulp mill from bark(4 MJ/ADMT) and black liquor (21 MJ/ADMT) fuels using alternative cogeneration technologies. Steam andelectricity demands (excluding powerhouse, Table 3.1) for alternative mill technologies are also shown. Thecogeneration technologies are the condensing extraction steam turbine (CEST) and black liquor/bark integratedgasification/gas turbine combined cycle (black liquor and bark are gasified separately). For the latter technology,two lines are shown. The upper line assumes the use of 8 :MJ/ADMT of forest or other biomass residues in additionto the 25 1vfJ/ADMT of fuels assumed for the lower line and for the CEST case. See Subbiah, et at (1995) fordetails of the calculations.

57

s. INDUSTRY ENERGY POLICY AND DECISION MAKING l

To identify effective approaches to improving energy efficiency that would also enhance the competitiveness of theindustry, an understanding of processes and technologies is necessary (Section 2-4). Equally important is someunderstanding of decision-making processes and strategies. The understanding of technology and decision-makingprovide a foundation for exploring energy efficiency strategies, including how and in which areas there might bebeneficial cooperation between the industry, utilities, government, regulatory agencies, and other institutions.

5.1. Energy decision making in the industryPurchased energy (Table 1.2) and capital investments to utilize these and self-generated biomass fuels represent majorproduction costs. The variety of corporate philosophies and structures across the industry preclude generalizationsabout how decisions are made, relating to these energy-related costs. Some companies use very decentralizedstructures, with even relatively major energy or non-energy related investment decisions made at the mill level. Inother cases, capital budgets and decision making are centralized and investments require approval from divisionand/or corporate headquarters. Many mill managers can decide on capital expenditures up to some limit and withina set budget, but both the limit and budget may vary with the economic climate (Aho and Boner, 1985). Severalcompanies have corporate energy management teams responsible for negotiating contracts with suppliers of fuels andelectricity. Many corporations have more or less explicitly formulated environmental goals, in some cases includingenergy strategies. Other corporations are in the process of developing such goals and strategies4

While generalizations are difficult, several observations relating to personnel and capital investment decision-makingare worth noting. An important lesson from the chernicals industry is that for the purpose of improving energyefficiency it is necessary to create an empowered efficiency culture in the company (Nelson, 1993). Also, whenupper management declares a company's serious commitment to the environment and to energy efficiency, the wordsmust be backed by commitment of resources.

Traditionally, pulp and paper companies have maintained skilled staff with responsibilities ranging from energypurchases to operation of on-site heat and power generation and energy demand management. However, a generaltrend in this and other industries is toward reduced staff dedicated to energy, with greater reliance on outsidespecialist consultants. One positive effect is that good ideas are spread more rapidly throughout the industry. Atthe same time, there arc fewer people on-site with the plant-specific experience that may be crucial to identifyingthe best site-specific opportunities to improve efficiency.

Energy related investments compete with other investments for capital. The relatively low fuel and electricity pricesprevailing since the mid-1980s have reduced the profitability of investmenL~ in energy efficiency improvements.Projects are generally ranked according to (1) profitability and (2) if they make the mill more competitive. Inpractice, this means that shorter paybacks are required for pure cost cutting measures, such as installing energyefficient motors or lighting, than for measures to increase production, improve product quality, increase market share,etc. However, many of the best opportunities for improving efficiency will also lead to improved productivity,quality, environmental performance't or other factors. For example, introduction of a variable speed drive might saveelectricity while affording better process control.

Investments that impact energy efficiency will tend to follow general invesunent patterns in the industry. The latterare highly cyclical (Nykanen, 1995). Annual capital expenditures in the U.S. pulp and paper industry reached $17billion in the 1989/1990 period, fell to about $10 billion in 1993, and are now on the upswing again. To someextent, this investment pattern aggravates the cyclic pricing of industry products, since capacity expansions can leadto over-capacity and hence falling product prices: high kraft market pulp prices in the late 1980s dropped to about$400 per tonne in the early 19905 and increased to more than $800 per tonne in the 1994/1995 period.

1 This chapter is panly based on discussions with representatives from International Paper, Georgia-Pacific Corp.,Weyerhaeuser Co., Union Camp Corp., Potlatch Corp., Stone Container Corp., CRS Sirrine Inc., Southern ElectricInternational Corp., Be Hydro, Minnesota Power, Bonneville Power Administration, and Barakat & Chamberlin.

58

5.2. Industry... utility cooperation in DSM programsElectric utility demand side management (DSM) programs aimed at load management and electricity conservationhave historically targetted the residential and commercial sectors. Industrial programs were introduced later. UtilityDSM program designs and experience vary from case to case (Jordan and Nadel, 1993).

A general concern with DSM are the consequences of rate increases that are needed for the utility to compensatefor lost revenue and program cost. While participating customers reduce consumption and as a result get lowerelectricity bills, non-participating ones may only see rate increases, if the DSM programs are not appropriatelydesigned. A number of approaches have been tried to deal with this concern. For example, industrial customers ofNiagara Mohawk can opt to be ineligible for financial DSM incentives and thereby avoid being allocated some ofthe program cost. For large industrial customers of Minnesota Power, 1.5% of the revenues from each customer areset aside by the utility for that customer to draw on when investing in energy efficiency.

In the preparation of this document, we undertook an informal sampling of pulp and paper companies to betterunderstand the extent of interest in utility DSM programs. All of the contacted companies have had some experiencewith utility sponsored programs, and several felt a useful role for utilities was in assisting mills to improve efficiencyin relatively generic end-use areas e.g., lighting, compressed air supply, pumping, and air-handling systems. At thesame time, most said they would rather have lower per-kWh electricity rates than utility-DSM programs. This maybe due to the perception that DSM programs contribute to higher overall electricity bills, because utility costs toadminister DSM programs are included in total prograln costs. (Companies feel they are able to more effectivelyspend funds for DSM measures on their own than through utility-related DSM programs.)

An ACEEE survey of industrial DSM programs (Jordan and Nadel, 1993) identified some common characteristicsof successful programs. These included understanding of the customer perspective, personal contact betweencustomer and utility, program flexibility, and financial incentives. Understanding the customer's perspective isclearly important in the pulp and paper industry where each corporation and individual mill may be different in termsof products, technology, markets, organization, etc. It is also important for the mill to have an appreciation of theutility's perspective. Such mutual understanding and trust is best developed through personal relationships betweenmill and utility representatives.

A good dialogue between utility and industry can also foster the development of more innovative and flexibleprograms that better suit the needs of each party. For example, one important lesson from an internal energyconservation competition at DOW Chemical is that nearly all winning projects involved modifications to processequipment, changes to the process itself, or to process operating strategies (Nelson, 1993). Almost none of theprojects involved areas typically covered in utility DSM programs, e.g. lighting and energy efficient motors.Minnesota Power's industrial DSM program provides some flexibility that addresses this shortcoming in traditionalDSM programs. For example one Minnesota Power customer used its set-aside funds to install a process controlsystem that yielded energy beneftis (among others).

Utilities are now increasingly downsizing industrial DSM programs, in particular by removing financial incentives,largely due to regulatory uncertainty with regards to increasing competition in the electricity sector. If a retailwheeling/direct access model being advocated by large users is adopted, then pulp and paper mills would get fullaccess to buying electricity at wholesale prices. This is likely to result in a withdrawal by utilities from DSMactivities and an increased emphasis on lower rates rather than lower bills. The final outcome, however, remainsuncertain, especially given the fact that critical decisions may well be made one state at a time.

S$3~ Cogeneration and other on-site utility systemsThe pulp and paper industry is distinguished from many other industries by the extensive use of black liquor andbiomass fueled cogeneration to provide much of on-site heat and power demands. There are also a number of otherutility systems on which the industry relies, including water supply, waste water treaunent, oxygen generators, andcompressed air systems. Changes to these utility systems are generally not given as high priority as product relatedprocess improvements, although financial risks associated with the fonner may be lower because such utility systemstend to have longer useful lives than process changes. In fact, it is not uncommon for companies to farm-out theoperation of mill utilities (typically to private independent companies), and in some cases such assets have been sold

59

to free up capital and to allow industries to concentrate on their core business--making paper.

Will the pulp and paper industry opt to make fuels and electricity production a part of their business or will otherparties own and operate the energy assets? Raymond (1993) reports that energy experts at some companies thinkthat within two decades many pulp mills will derive as much value from energy ventures as from the pulp itself.At another extreme, some companies arc selling off their entire energy cOlnplexes to free up capital that can bereinvested in the core business.2 In another case, plans are being developed between Virginia Power and theChesapeake Corporation for Virginia Power to build, own and operate a natural gas fired cogeneration plant atChesapeake's West Point min to replace an old fossil fuel fired boiler.

Since power is their core business, independent power producers or utilities could conceivably be more effective thanpaper companies at operating cogeneration plants as profit centers, and they might be better positioned to adopt newtechnology. Oxygen is another case where it can make sense, if oxygen demand is high, for a pulp mill to buy overthe fence rather than own and operate its own equipment. This is done, for example, at Chesapeake's West Pointmill, where an oxygen supplier combines the production of oxygen next to the mill with the production of othermarketable gases. Depending on contract arrangements, however, there could be disincentives to improving energyor overall resource efficiency if the ownership/operation and the end-usc of such mill utilities are separated. Forexample, if a utility (compressed air, oxygen, etc.) were to be provided at a flat rate, or under a take-or-pay contract,the user would have little incentive to reduce consumption.

It remains to be seen to what extent there will be a market for electric utilities, independent power producers, andothers to own and operate cogeneration and other utilities plants adjacent to pulp and paper mills, as well as whatthe impact of different ownership structures will be on energy efficiency in the industry.

5..4. Industry...government cooperationThe pulp and paper industry has not participated in federally funded industrial research to the same extent as manyother industries. However, guided by existing industry-government partnerships in the semiconductor, textiles, andautomotive industries, the U.S. Department of Energy and the American Forest and Paper Association announceda major new joint research initiative in November 1994 ainled at promoting industrial growth, energy efficiency, andinternational competitiveness, while preserving the environment (AFPA, 1994c). The next step is to develop anoperating structure, a framework for the partnership, and a blueprint for the implclnentation program and evaluationof results. An important objective of establishing this voluntary collaborative effort is to leverage industry R&Dspending through identifying appropriate areas for joint government-industry R&D and thereby insure that limitedfunds are spent strategically. Product related R&D is not included in the agreement since it is the basis forcompetition between companies and is best left to individual efforts of proprietary R&D programs.

The six key areas identified in Agenda 2020 are sustainable forest management, environmental performance, energyperformance, improved capital effectiveness, recycling, and sensors and controls. Specific priorities within the energyand environment areas include reduction of gaseous, liquid and solid waste emissions, non-process element removal,drying technologies, and gasification and cogeneration tcchnologies.

5~5o Standards and eco.. labelsThe pressures on industry to improve environmental pcrformanceis also manifested in the development of eco­labelling, international standards, and changing markets. For example, L.L. Bean and IKEA catalogues, Ocr Spiegelin Germany, and several other publications are now printed on chlorine free or recycled paper. The EPA has issuedguidelines that specify the amount of recycled fiber from post consumer waste that should go into different paperand paperboard products procured by government and federal agencies (EPA, '199_). Such policies may be effective

2 Scott Paper recently sold off the energy complexes at Mobile, Alabama and Chester, Pennsylvania mills toindependent power producers Southern Electric International and CRSS Inc., respectively (Scott Paper, 1994; ScottPaper, 1995; CRSS, 1995). The energy complex in Mobile includes two new recovery boilers. As part of theagreement, the mills will purchase steam and electricity from these companies.

60

in addressing some environmental problems, e.g., by reducing the amount of municipal solid waste going to landfills,but may create others, e.g., increased fossil fuel use and related emissions due to recycling. Careful life-cycleanalyses (LeA) can help address such problems (Gilbreath, 1995), and the acceptance for such policies can besubstantially improved by ensuring industry participation in the development of guidelines.

The role of international environmental standards, such as ISO 14000 which is now being developed, is to certifythat a company has an environlnental policy. For a cOlnpany to get certified it must be able to show that it has anenvironmental management system and that it is being implemented. The earlier standard ISO 9000, requires qualitymanagement systems. Buyers of various products, for example, aUlO manufacturers, may require that companies areISO certified in order for them to qualify as suppliers. A standard such as the ISO 14000 is relatively weak in thatthe subject company specifics the environmental policies and goals. The strength is that the company is checkedregularly and loses certification (and thus risks losing customers) if it does not implement the policies and meetstated goals. Certifying agencies are, for example, Norske Veritas and Lloyds.

Eca-Iabels usually have quantitative requirements as to product content and production method. Environmental andgovernmental organizations in Europe appear to have been more active in developing eco-Iabels than Americanorganizations. An important feature of many of these labels are that they have been developed cooperatively byenvironmental organizations, industry and government agencies. Such labels include the Nordic Swan, the BlueAngel in Germany't and Green Seal in the U.5. (Richards, 1994). Eco-labels complement environmental managementsystem standards since they are used primarily for consulncr products, such as detergents, paper, and foods. Theyare likely to play an increasingly important role as consumer awareness of environmental issues increases.

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6. FUTURE DIRECTIONSTo conclude, we discuss some possible future directions in energy efficiency in the U.S. pulp and paper industry.Energy efficiency in the context of this industry (unlike most industries) must include, in addition to processenergy use, consideration of how the biomass-derived fuels that are available at many mill sites are used.

As discussed in earlier sections of this document, there are potentially major opportunities for improving theefficiency of process energy usc. For example, comparisons of specific energy use in the U.S. industry againstthat in Scandinavia, where historically greater emphasis has been placed on minimizing process energy use,demonstrate that large reductions in energy use are technically possible. While there are important opportunitiesfor improving efficiencies of equipment found throughout all industry (e.g. pumps, fans, etc.), the bestopportunities will typically involve process-related changes. A number of new energy-saving processtechnologies, e..g. digesters and paper or pulp dryers, are under development or recently commercialized, andprocess heat integration (pinch) analysis has been applied in a handful of mills. Undertaking to capture theenergy savings associated with process-related changes is made more difficult by the generally capital-intensivenature of the required investments. Utility or government-backed energy efficiency programs or policies aremore likely to be successful if they address this issue specifically. On the other hand, most process-specificchanges that bring energy efficiency improvements also bring productivity or other improvements, so that suchinvestments need not be justified on the basis of energy savings alone.

Measuring (or score-keeping) energy savings and the related return on investments has been demonstrated inother industries and sectors to be crucial to succeeding in capturing energy savings. At present there is nosystematic reporting of energy use in the pulp and paper industry, except total industry energy use, as reportedby the American Forest and Paper Association based on survey data. These data are useful for tracking nationaltrends, but not for assessing product-specific energy efficiency performance. Sweden's Forest IndustriesAssociation regularly reports the highest, average and lowest energy use per tonne of product for some keyproducts (without disclosing proprietary information), which provides benchmarks against which individual millscan calibrate their energy performance. The U.S. industry might consider such an approach. With greateremphasis on tracking energy use at the mill level, companies would be able to better evaluate energyperformance over time and between different mills within the company.

Reducing process energy needs per tonne of product might have some important cascade effects. It should leaddirectly to reduced environmental impacts. It .night also facilitate quantum improvements in cogenerationtechnologies for on-site heat and power generation. The pulp and paper industry is in a unique and enviableposition of being able to rely on il~ own internally-generated fuels from renewable biomass sources for morethan half of its process energy requirements. In the relatively near future, the industry has the potential toprovide essentially all of the energy requirements needed at many of its facilities, and perhaps even exportbiomass-generated power to other users.

Advanced biomass-gasifier and black-liquor gasifier/gas turbine power systems, which would provide quantumimprovements in cogeneration efficiency over existing boiler-steam turbine technology, are undergoing rapiddevelopment. Such systems are likely to be commercially ready by around the tum of the century. The pulpand paper industry is a prime initial market for such systems because it has biomass fuels available on-site andbecause it will be retiring much of its existing biomass-cogeneration equipment during the next decade or so.Together with advanced biomass-cogeneration technologies, competition in wholesale electricity markets maycreate opportunities for the industry to market renewable-electricity directly. The industry might alternatively optto divest its energy assets and primarily be a supplier of biomass fuels without adding much value to them. Theindustry would likely benefit in either case, so their taking a leadership role in developing and commercializingsuch technologies would seem entirely appropriate.

Finally, environmental issues will continue to playa key role in driving technology development and relatedenergy use in the pulp and paper industry. These issues might be most effectively addressed to the benefit of theindustry ilirough partnerships with government, utilities, and non-governmental organizations. The Agenda 2020initiative (AFPA, 1994c), which targets the development of environmentally-responsive, energy-relatedtechnologies and processes (among a select number of other areas) is a useful initial step in this direction.

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